Astronomical Almanac Description

The Astronomical Almanac is an interface to the Sky Calculator programme written by John Thorstensen of Dartmouth College. The code precesses coordinates, gives object positions, time, date, Sun, Moon and planet information. This document gives some brief instructions and guidelines to help you get some information back from the Astronomical Almanac. You will also find very detailed descriptions of the input and output parameters for this facility.

(1) Brief Instructions for the Astronomical Almanac

1. CHOSE YOUR LOCATION:
   • select a site from the "Location" menu, or
   • select "Use Inputs" from the "Location" menu and provide the latitude, longitude and height in the input fields below.
2. CHOOSE YOUR OBJECT:
   • select an object from the "Astronomical Object" menu, or
   • select "Use Specified Catalogue", choose a catalogue from the "Catalogue" menu and input the name of your object in the "Catalogue Object" input field, or
   • select "Use Inputs" from the "Astronomical Object" menu and provide the RA (right ascension), declination and epoch in the input fields below.
3. CHOOSE YOUR DATE AND TIME:
   • select "Time Now" from the "Time Type" menu and choose a Daylight Saving Time (DST) option from the "DST" menu, or
   • select "Local Date" or "Literal Date" from the "Date Type" menu, select "Local Time" or "UT Time" from the "Time Type" menu, input the date, time and time zone in the input fields above, and choose a Daylight Saving Time (DST) option from the "DST" menu.
4. SUBMIT YOUR REQUEST:
   • Click on the "Submit Request" button.

(2) Guidelines

1. Only spaces should be used to seperate numbers within the input fields, not, for example, colons.
2. Latitude is in degrees, minutes and seconds and should not be preceded by a positive sign if it is a North latitude.
3. The longitude must be given in hours, minutes and seconds not degrees, minutes and seconds.
4. The height should be in metres. If it is not known then interpret the results as at sea level for the specified location.
5. Searching the catalogues for a named object can be a time-consuming approach and should only be used if the correct coordinates are unknown.
6. It would be sensible if there is a reasonable chance that your object will be found in the selected catalogue and that its name is standard.
7. Be sure to select the correct date type. The "Local Date" option specifies a local date for evening. This means that a time specified in the morning (e.g. 02 00 00 hours) is assumed to occur during the morning after the date you specify. The other option of "Literal Date" means the date will be interpreted literally so that phenomena reported will be for the night of that date and the following morning. Note that selecting "UT Time" from the "Time Type" menu automatically sets the date to be interpreted literally.

(3) Description of the Astronomical Almanac Input Parameters

Location

This is the site name of the location on Earth that has been chosen for which to perform the calculations. The pull down menu allows the choice of some of the more important VLBI sites around the world, including the EVN. If one of these sites is chosen from the menu then its latitude, longitude and height will be automatically used during the calculations. If the option "Use Inputs" is selected the site location is that given in the "Latitude", "Longitude" and "Height" fields. In this case the site name will be "Unknown".

Astronomical Object

This is the name of the astronomical object that has been chosen for which to perform the calculations. The menu allows the choice of some of the more important radio astronomical calibration sources. If one of these objects is chosen from the menu then its correct right ascension, declination and epoch will be automatically used during the calculations. If the option "Use Inputs" is selected the object coordinates are those given in the "RA", "Declination" and "Epoch" fields. In this case the object name will be "Unknown". If the "Use Specified Catalogue" option is selected the Astronomical Almanac will search the catalogue of objects selected in the "Catalogue" menu for the object specified in the "Catalogue Object" field. If found the object's coordinates will be used automatically in the calculations.

Catalogue

There are many catalogues of astronomical objects which give details of coordinates. The Astronomical Almanac allows access to some of the more important catalogues for obtaining those coordinates. The "Catalogue" menu allows the user to specify the catalogue in which the Astronomical Almanac should search for the object specified in the "Catalogue Object" field. This search will only be performed if the "Use Specified Catalogue" option is selected in the "Astronomical Object" menu. An error will be returned if the specified object is not found in the specified catalogue. If found the object's coordinates will be used automatically in the calculations. Note that searching the database of catalogues for a particular object may be a time-consuming approach and should only be performed if the correct coordinates of the object are unknown. At present the available on-line catalogues are as follows;

• NGC/IC Catalogue - The New General Catalogue and Index Catalogue
• Messier Catalogue - The Messier Catalogue of Nebulae and Galaxies

Catalogue Object

This specifies the object name which the Astronomical Almanac should attempt to locate in the catalogue specified by the "Catalogue" menu. This search will only be attempted if the "Use Specified Catalogue" option is selected in the "Astronomical Object" menu. An error will be returned if the specified object is not found in the specified catalogue. If found the object's coordinates will be used automatically in the calculations. Note that searching the database of catalogues for a particular object may be a time-consuming approach and should only be performed if the correct coordinates of the object are unknown.

Latitude

This is the geographic latitude of the location for which to perform the calculations. This coordinate will only be used if the "Use Inputs" option is selected in the "Location" menu. The latitude is the angular distance of a point on the Earth's surface north or south of the equator, measured as the angle subtended at the centre of the Earth by an arc along a meridian between the point and the equator. Latitude is measured in degrees, minutes and seconds. Positive latitudes are north of the Earth's equator, negative ones are south of the equator. The equator is at latitude 0 degrees while the north and south poles are at latitudes of +90 and -90 degrees respectively. For the Astronomical Almanac specify the latitude in degrees, minutes and seconds, omitting the positive sign for positive latitudes.

West Longitude

This is the geographic West longitude of the location for which to perform the calculations. This coordinate will only be used if the "Use Inputs" option is selected in the "Location" menu. The longitude is the angular distance of a point on the Earth's surface east or west of a central meridian, measured by the angle between the plane of the meridian through the point and that of the central meridian. By international agreement the standard meridian for longitude passes through Greenwich, England. Longitude is measured in degrees, minutes and seconds so that the Greenwich meridian is at 0 degrees longitude. In passing through 15 degrees of longitude at the Earth's equator there is a time difference of one hour so that longitude can also be expressed in hours, minutes and seconds. This is the requirement for the Astronomical Almanac. To convert longitude in decimal degrees to decimal hours divide by 15. Then express the result in hours, minutes and seconds. The Astronomical Almanac requires West longitudes so that East longitudes must be inputted as negative numbers.

Height

This is the geometric height above mean sea level (measured in metres) of the location for which to perform the calculations. This height will only be used if the "Use Inputs" option is selected in the "Location" menu. The height of the site makes a difference to the times that phenomena such as sunrise occur for the observer. Height is also sometimes called altitude but in astronomy altitude is also synonomous with elevation. To avoid confusion we use the terms height and elevation. For the Astronomical Almanac specify the height above mean sea level for your location in metres. If you do not specify a height, sea level will be assumed, and the results could therefore be approximate for your location.

Date

This is the date for which you wish the calculation to be performed. It should be specified as three numbers, the year (e.g. 1997), the month number (e.g. 9 for September) and the day of the month. Note that there are two methods by which you can specify the date (selected using the menu for "Date Type").

Time

This is the time for which you wish the calculation to be performed. It should be specified as three numbers, the hour, minutes and seconds on the 24 hour clock (e.g. 21 30 20). Note that there are several methods by which you can specify the time (selected using the menu for "Time Type").

Time Zone

The time zone in which the location for the observations is situated makes a difference to local times of midnight and so on. The time zone can be specified to account for these differences. Note that the time zone is positive westwards which is not the usual convention. Hence a time zone 5 hours behind Greenwich Mean Time has a positive time zone. The time zone should be inputted as a single number between 0 and 24.

RA (Right Ascension)

The right ascension (RA) of an object in the sky is its angular distance measured eastwards along the celestial equator from a catalogue equinox to the intersection of the hour circle passing through the body. The point from which RA is measured on the sky, which is analogous to the Greenwich meridian in the terrestrial coordinate system, is called the vernal equinox and is the point in the sky where the Sun moves from positive to negative declination during the course of its annual motion through the sky. Right ascension is the equivalent to terrestiral longitude on the celestial sphere (rather than the Earth) in the equatorial coordinate system. It is normally measured in hours, minutes and seconds but like terrestrial longitude can also be expressed in degrees. One hour of RA at the celestial equator is equivalent to 15 degrees. Together with declination the RA specifies the position of an object in the sky irrespective of the observer. The local sidereal time (LST) is the right ascension of the zenith (the point in the sky directly overhead). For the Astronomical Almanac specify the target RA in hours, minutes and seconds at the epoch given. This coordinate will only be used if the "Use Inputs" option is selected in the "Astronomical Object" menu. If a given source is selected from this menu or the "Use Specified Catalogue" option is selected the Astronomical Almanac will automatically use the correct coordinates of the source.

Declination (Dec)

The declination of an object in the sky is its angular distance from the celestial equator measured along the hour circle passing through the object. Declination is the equivalent of terrestrial latitude on the celestial sphere in the equatorial coordinate system and is measured in degrees, arcminutes and arcseconds. There are 60 arcseconds to an arcminute and 60 arcminutes to a degree. Just like terrestrial latitude the declination ranges from +90 degrees at the North Celestial Pole (NCP) to -90 degrees at the South Celestial Pole (SCP). Together with right ascension the declination specifies the position of an object in the sky irrespective of the observer. The declination of the zenith (the point in the sky directly overhead) is equal to the latitude of the observer on the Earth. For the Astronomical Almanac specify the target declination in degrees, arcminutes and arcseconds at the epoch given. This coordinate will only be used if the "Use Inputs" option is selected in the "Astronomical Object" menu. If a given source is selected from this menu or the "Use Specified Catalogue" option is selected the Astronomical Almanac will automatically use the correct coordinates of the source.

Epoch

The epoch is a precise instant that is used to fix a reference frame for astronomical coordinates. For example the right ascension and declination of an object are continuously changing (although slowly) due to the precession of the equinoxes (a result of the wobble of the Earth as it spins on its axis). Coordinates must therefore be referred to a particular epoch or instant in time when they are correct. This epoch can be the time of observation, the beginning of a particular year or the beginning of a half-century. The standard epoch specifies the reference system to which coordinates are referred. Before 1984 the coordinates of objects were normally referred to the mean equator and equinox of the beginning of a Besselian year (for example B1950.0). Since 1984 the standard epoch has been the Julian year and the most common epoch is January 1.5 2000 AD (indicated by J2000.0). The Astronomical Almanac recognises Besselian and Julian epochs with no preceding character. The most common epochs to use are 2000.0, 1950.0 or the epoch of the current date. The epoch of the RA and declination coordinates is only used if the "Use Inputs" option is selected in the "Astronomical Object" menu.

Date Type

This menu allows you to tell the Astronomical Almanac how to interpret the date you have given it. The "Local Date" option (default) makes the Astronomical Almanac assume the date you give is the local date for evening. This means that a time specified in the morning (e.g. 02 00 00 hours) is assumed to occur during the morning after the date you specify. This is so the information given applies to a single night of observations as is often the case for optical observing. The other option of "Literal Date" makes the Astronomical Almanac interpret the date as that literally given so that phenomena reported will be for the night of that date and the following morning. Be careful in the interpretation of the results when specifying the date type. Note that selecting times in Universal Time automatically sets the date type to literal since UT times are always interpreted as such.

Time Type

This menu allows you to tell the Astronomical Almanac how to interpret the time you have given it. The "Local Time" option (default) makes the Astronomical Almanac assume the date you have given it is the local time. Exactly how this is interpreted also depends on the date type you are using, "Local Date" or "Literal Date". The "UT Time" option specifies that the time you are sending is in Unversal Time. This automatically sets the date type to literal since UT times are always interpreted as such. The final option "Time Now" allows you to bypass completely the time setting and make the Astronomical Almanac use the actual time it receives the request.

DST Use (Daylight Saving Time)

The DST Use menu allows you to use one of the standard Daylight Saving Time schemes around the world. These systems make changes to the local time during certain times of the year to shift the daylight hours more conveniently onto the local time. The use of DST can be switched off using the "None" option. If another option is selected the local time for the location will be reported as a DST. Currently the standard DST systems possible for the Astronomical Almanac are US (starts first Sunday in April after 1986 or the last Sunday in April before 1986, ends last Sunday in October), European (starts last Sunday in March, ends last Sunday in September), Chilean (starts second Sunday in October, ends second Sunday in March) and Australian. All time changes are assumed to occur at 2 am as reckoned in the time preceding the change. If in doubt about the use of DST select "None".

(4) Description of the Astronomical Almanac Output Parameters

Precessed RA (right ascension)

Presession is the slow periodic change in the direction of the Earth's rotation axis caused primarily by the Sun and Moon's gravitational attraction on the equatorial bulge. This results in the celestial poles tracing out a circle in the sky of about 23.5 degrees in radius once every 25800 years or so (this is called the Platonic year). The Earth's precession leads to the precession of the equinoxes. The equinoxes are the two points on the celestial sphere where the ecliptic intersects the celestial equator. The ecliptic is the mean plane of the Earth's orbit around the Sun and hence the apparent annual path of the Sun through the sky. The vernal equinox (now properly called the dynamical equinox) is the point where the Sun passes from the south to the north celestial hemisphere and is used as the reference point for right ascension measurement. The sun passes from the north to the south celestial hemisphere at the autumnal equinox. Because the equinoxes are not fixed in position but move westward around the ecliptic due to precession the right ascension and declination of an object change with time. The precessed right ascension is the right ascension of the object at a given instant in time (the precessed epoch) as opposed to the right ascension at the standard epoch. The Astronomical Almanac reports the precessed right ascension of the object in hours, minutes and seconds for the epoch of observation.

Precessed Dec (declination)

Just as the right ascension of an object changes with time due to the precession of the equinoxes so too does the object's declination. The precessed declination is the declination of the object at a given instant in time (the precessed epoch) as opposed to the declination at the standard epoch. The Astronomical Almanac reports the precessed declination of the object in degrees, arcminutes and arcseconds for the epoch of observation.

Precessed Epoch

The precessed epoch is the epoch of observation to which the precessed right ascension and declination of the object refer. It is expressed as a decimal year like the standard epoch, the decimal quantity being the fraction of the current year elapsed. The Astronomical Almanac reports the precessed epoch in decimal years.

Parallactic Angle

The parallactic angle of an object is the angle between the celestial pole (north or south depending on location), the object and the zenith (the point in the sky directly overhead). It can also be described as the position angle of a great circle connecting the object to the zenith. The parallactic angle can be an important quantity in some observations because it describes the orientation on the sky of the object for a particular observer. The Astronomical Almanac reports the parallactic angle of the object in degrees and (in square brackets) the antiparallel angle (plus or minus 180 degrees).

Elevation

The elevation of an object in the sky is its angular distance above the observer's horizon. Elevation is therefore a measure of how far up in the sky the object is located. It is measured in degrees, arcminutes and arcseconds with the horizon at 0 degrees and the zenith (the point in the sky directly overhead) at +90 degrees. An object with a negative elevation is below the observer's horizon. The point in the sky directly "below" the observer's feet (known as the "nadir") is at an elevation of -90 degrees. Lines in the sky of constant elevation for a particular observer are called "almucanters". Together with azimuth the elevation specifies the position of an object in the sky for a particular observer in the horizon coordinate system. Since the sky (or in fact the Earth) rotates, the elevation of an object is constantly changing and depends on the observer's location on Earth and the time of observation. Elevation is sometimes called altitude but this is more commonly used to describe the height in metres above sea-level. To avoid confusion we use the terms height and elevation. The Astronomical Almanac reports the target's elevation in decimal degrees.

Azimuth

The azimuth of an object in the sky is the angular distance measured eastwards along the horizon from the north point to the intersection of the object's vertical circle (the line drawn from the zenith (the point in the sky directly overhead) through the object to the horizon). It is measured in degrees, so that north is 0 degrees, east is 90, south is 180 and west is 270 degrees. If azimuth is negative then it is measured westwards from the north point. It is also possible that azimuth may be expressed as an angle from the south point on the horizon but this is less common. However, for an observer in the southern hemisphere the azmiuth is measured from the south point eastwards. Together with elevation the azimuth specifies the position of an object in the sky for a particular observer in the horizon coordinate system. Since the sky (or in fact the Earth) rotates, the azimuth of an object is constantly changing and depends on the observer's location on Earth and the time of observation. The Astronomical Almanac reports the target's azimuth in decimal degrees.

Hour Angle

The hour angle of an object in the sky is the angle measured westwards along the celestial equator from the observer's meridian (a line drawn from the zenith (the point in the sky directly overhead) to the south point on the observer's horizon) to the hour circle (a line passing through the object's position and the north and south celestial poles) of the object. It is usually expressed in hours, minutes and seconds and has the same units but opposite direction to right ascension. The angle measured eastwards along the equator from the meridian is sometimes called the meridian angle. The hour angle of an object is 0 hours when it is due south of the observer, is positive when the object is in the west and negative when it is in the east. The local hour angle of an object is the local apparent sidereal time minus the apparent right ascension. The local sidereal time is the local hour angle of a catalogue equinox so an object crosses the local meridian when the local sidereal time is equal to the object's right ascension. The Astronomical Almanac reports the target's hour angle in hours, minutes and seconds.

sec z (air mass)

Most astronomical observations from the ground are affected by the Earth's atmosphere which disperses, absorbs and refracts the radiation arriving from the object under study. The degree to which the atmosphere affects the observations can depend on many things, for example, the altitude or humidity, but in all cases is worse when the object is low down in the sky. This is because the radiation is travelling through a thicker layer of atmosphere if it is low down in the sky. One way of characterising the effect of the atmosphere is to therefore state how high up the object is, or alternatively, how low down it is. Thus the zenith distance or coaltitude (given the symbol z) is an appropriate measurement. The zenith distance is the angular distance of the object from the observer's zenith measured along the vertical cricle passing through the object. It is therefore the complement of the elevation (i.e. it is 90 degrees minus the elevation). Because the effect of the atmosphere does not double for double the zenith distance (it is not a linear effect) astronomers often use the secant of the zenith distance (the secant is the reciprocal of the sine of the angle) or "sec z". This is the quantity which specifies how much atmosphere the radiation has traversed and is often called the airmass. Actually, the actual airmass is not equal to sec z but only differs greatly from it very near the horizon. The smaller the value of sec z the higher the object in the sky. The higher the value of sec z the greater the effect of the Earth's atmosphere will be. The almanac reports the airmass or sec z for the object. If sec z is very large or the object is below the horizon the almanac reports sec z to be "large".

Galactic Latitude (Gal. b)

The galactic coordinate system is commonly used to study the structure and surroundings of the Galaxy. It is defined by a fundamental circle along the galactic equator (the great circle on the celestial sphere which represents the plane of the Galaxy) with the zero point towards the centre of the Galaxy in the constellation of Sagittarius. The galactic plane is the plane that passes most nearly through the central plane of the spiral disc of the Galaxy. The galactic latitude (b) of an object in the sky is its angular distance north or south of the galactic equator. Galactic latitude runs from -90 degrees at the south galactic pole, through 0 degrees at the galactic equator to +90 degrees at the north galactic pole. It is measured along the great circle passing through the object and the two poles. Together with galactic longitude (l) the galactic latitude defines the position of an object in the galactic coordinate system. The galactic plane and the celestial equator are inclined at an angle of about 62 degrees. The Astronomical Almanac reports the galactic latitude of the object in decimal degrees.

Galactic Longitude (Gal. l)

The galactic longitude (l) of an object in the sky is its angular distance (from 0 to 360 degrees) from the nominal galactic centre measured eastwards along the galactic equator to the intersection of the great circle passing through the object. The position of zero galactic longitude (the galactic centre) is at RA 17:42.4 and declination -28:55. Together with galactic latitude (b) the galactic longitude defines the position of an object in the galactic coordinate system. The Astronomical Almanac reports the galactic longitude of the object in decimal degrees.

Ecliptic Latitude (Ecl. lat.)

The ecliptic is the mean plane of the Earth's orbit around the Sun. The ecliptic is therefore a circle on the sky which defines the Sun's apparent annual path across the sky. The orbits of the Moon and planets, apart from Pluto, as seen from Earth, lie very near to the ecliptic. The planes of the ecliptic and celestial equator are inclined at an angle equal to the tilt of the Earth's rotation axis. This angle is known as the obliquity of the ecliptic and is about 23.5 degrees. The intersection of the ecliptic with the celestial equator define the equinoxes. The poles of the ecliptic are at RA 18 hours, declination +66 degrees and RA 6 hours, declination -66.5 degrees in the celestial coordinate system. The ecliptic and the ecliptic poles define the basis of the ecliptic coordinate system which is often used to describe the positions of Solar System bodies. The ecliptic latitude of an object in the sky is its angular distance north or south of the ecliptic. Ecliptic latitude runs from -90 degrees at the south ecliptic pole, through 0 degrees at the ecliptic to +90 degrees at the north ecliptic pole. It is measured along the great circle through the object and the poles of the ecliptic. Together with ecliptic longitude the ecliptic latitude defines the position of an object in the ecliptic coordinate system. The Astronomical Almanac reports the ecliptic latitude of the object in decimal degrees.

Ecliptic Longitude (Ecl. long.)

The ecliptic longitude of an object in the sky is its angular distance (from 0 the 360 degrees) measured eastwards along the ecliptic from the position of the vernal equinox to the intersection of the object's great circle of longitude. It is measured in the same direction as the Sun's apparent annual motion. Together with ecliptic latitude the ecliptic longitude defines the position of an object in the ecliptic coordinate system. The Astronomical Almanac reports the ecliptic longitude of the object in decimal degrees.

Local Date

The Astronomical Almanac reports the local date details for the date and time specified and includes the day of the week, the year, the month and date.

UT Date

The Astronomical Almanac reports the UT date details for the date and time specified and includes the day of the week, the year, the month and date. These details are given because the UT date can be different to the local date because local date is location dependent whereas UT date is not.

Local Time

The Astronomical Almanac reports the local time for the location of the observation that corresponds to the time of observation whether that has been specified as a local time or a UT time.

UT Time

Universal Time (UT) is a precise measurement of time which forms the basis of all civil timekeeping. It is determined by precise observations of the diurnal (daily) motion of stars. Due to variations in the Earth's rotation UT is not a uniform timescale. UT0 is the UT timescale derived from observations and is location dependent whereas UT1 is the timescale is that obtained by correcting for the variation in the observer's meridian that results from the irregular varying motion of the Earth's rotation axis. Coordinated UT (UTC) is based on the International Atomic Time (TAI) and minimizes the divergence of UT from the uniform atomic timescale. UTC differs from TAI by an integral number of seconds and is kept to within 0.9 seconds of UT1 by the insertion or deletion of a single leap second usually at the end of December or June. The Astronomical Almanac reports the time of the observation in UT.

LMST (Local Mean Sidereal Time)

The local mean sidereal time is the hour angle of the vernal equinox at a particular instant. The Astronomical Almanac LMST is slightly different to this because the effect of the nutation of the Earth's orbital axis is not taken into account. Nutation is a slight periodic but irregular movement of the Earth's rotational axis superimposed on the precessional motion. It is caused by the varying distances and relative directions of the Moon and Sun. Although a small effect it is important in highly accurate calculations. The complete correction for precession and nutation is called the equation of the equinoxes. Generally this correction will be less than two seconds in time. However, if the input time to the Astronomical Almanac is given in Coordinated Universal Time UTC (a UT time system tied to atomic clock measurements and the basis of all civil time systems) then the LMST returned will be UT - UTC which is less than a second.

LMST at Midnight

The Astronomical Almanac reports the local mean sidereal time at the instant that the vernal equinox passes through an hour angle of zero, i.e. the LMST is given for the instant of midnight for the chosen day.

DST

The Astronomical Almanac reports whether the calculations have been performed with a Daylight Savings Time (DST) in effect. If a DST option has not been chosen it will report "None" otherwise it will report which option has been chosen, "US", "European", "Chilean" or "Australian".

DST time

The Astronomical Almanac reports the time at which DST comes into effect (currently 2 am). If one has not been chosen "none" will be reported.

DST Start

The dates at which Daylight Savings Time (DST) comes into operation for each DST option are currently set by convention within the Almanac and cannot be set by the user. The Astronomical Almanac therefore reports the date at which the DST option comes into effect. If a DST option is not selected "none" will be reported.

DST End

The dates at which Daylight Savings Time (DST) ceases operation for each DST option are currently set by convention within the Almanac and cannot be set by the user. The Astronomical Almanac therefore reports the date at which the DST option ceases. If a DST option is not selected "none" will be reported.

Julian Date

The fact that our calendars have changed over the years, and have changed in different parts of the world at different times, means it is sometimes difficult to be absolutely definite about the date on which an event occured. To alleviate this problem astronomers use Julian dates which form a consecutive day numbering system which is universal and unambiguous. The Julian date is the number of days elapsed since noon Greenwich Mean Time on 1st January in the year 4713 BC. This rather obscure date arises because of the definition made by Josephus Justus Scalinger in the sixteenth century of the Julian period, equal to 7980 Julian years. The Julian year is equal to exactly 365.25 days each containing exactly 24 hours and was introduced as the basis of the Julian calendar in 46 BC by the Roman Emperor Julius Caesar. The Julian period is the least common multiple of the 28 year solar cycle, the 19 year lunar cycle and an ancient non-astronomical cycle of 15 years known as the cycle of the indiction. The year 4713 BC is the year when all these cycles began together. Julian dates are decimals, the decimal part giving the fraction of a day elapsed since the preceding noon. The exact time of noon on a given day will have the decimal part of the Julian date equal to 0 while exact midnight will have a decimal part of 0.5. For example, the Julian date at noon UT on 1st March in the year 2000 will be 2451605.000 while the midnight following will have a Julian date of 2451605.500. The Astronomical Almanac reports the exact Julian date of the date and time used in the calculations.

Barycentric Julian Date

When astronomers wish to compare the times of occurrence of astronomical events at different locations, for example, the times of arrival of pulses of radiation or particles or the precise moment an eclipse begins, they must take into account the different distances from the source of the phenomenon to the point of its observation. Light, for example, may take a little longer to reach someone observing a phenomenon at a different location. To alleviate this kind of problem astronomers can define a standard point in space and express times of occurrence as observed from that point. One of the most common points of reference is the barycenter (the center of mass) of the solar system. The Astronomical Almanac reports the Barycentric Julian Date which is the Julian Date at the date and time specified after accounting for the light travel time between the location of the observation and solar system barycenter. Note that the point of observation is taken as the position of the Earth's center of mass and the time of flight across the Earth's radius is not included in the calculation.

Local Midnight

The Astronomical Almanac reports the date and UT time on which the local midnight occurs at the location of observation and includes the year, month, date and hour.

Midnight Julian Date

The Astronomical Almanac reports the Julian Date of the moment of midnight for the date and time specified. By definition the Julian Date at midnight has a decimal part of 0.5 since Julian days begin at midday rather than midnight.

Sun

The Astronomical Almanac reports a very simple description of the Sun's position at the time and date specified. It is either "UP" or "DOWN".

Barycentric Seconds Correction

The Barycentric Seconds Correction is the difference between the Julian Date and the Barycentric Julian Date for the date, time and location of observation, measured in seconds. It represents the light travel time between the location of observation and the barycenter of the solar system. Note that the point of observation is taken as the position of the Earth's center of mass and the time of flight across the Earth's radius is not included in the calculation. However, the time should be accurate to within 0.2 seconds.

Barycentric Velocity Correction

Just as astronomers may wish to compare the times of occurrence of astronomical events at different locations, they may also wish to compare the velocities of objects measured from different locations or at different times. To do this they must take into account the velocity of the Earth with respect to the object which changes with location and with time because of the rotation of the Earth and its orbit around the Sun etc. To alleviate this problem astronomers can correct measured velocities to be those that would be measured at the same point in space. A common point of reference is the barycenter (the center of mass) of the solar system. The Astronomical Almanac reports the Barycentric Velocity Correction which is the velocity (in km/second) to be added to any velocity measured at the specified date and time to give the appropriate velocity at the barycenter of the solar system. Note that the point of observation is taken as the position of the Earth's center of mass.

Sun RA (right ascension)

The Astronomical Almanac reports the right ascension (RA) of the Sun for the date and time specified.

Sun Dec (declination)

The Astronomical Almanac reports the declination (Dec) of the Sun for the date and time specified.

Sun El (elevation)

The Astronomical Almanac reports the elevation (El) of the Sun for the date and time specified. The elevation is negative if the Sun is below the horizon.

Sun Az (azimuth)

The Astronomical Almanac reports the azimuth (Az) of the Sun for the date and time specified.

Sunset

Sunset is defined as the evening time at which the apparent upper limb of the Sun is on the astronomical horizon. The Astronomical Almanac reports the sunset time for the date and time given in Zone Standard Time (ZST).

Sunrise

Sunrise is defined as the morning time at which the apparent upper limb of the Sun is on the astronomical horizon. The Astronomical Almanac reports the sunrise time for the date and time given in Zone Standard Time (ZST).

Darkness

The Astronomical Almanac reports the total number of hours that the Sun is below the horizon.

Duration

The duration of the night is the total number of hours between evening and morning astronomical twilight.

Eve Twilight

Twilight is a term applied to the time preceding sunrise and following sunset during which the sky is partially illuminated. The times of twilight are important for astronomers because it indicates exactly when the sky will become dark enough for them to observe faint objects. There are actually three kinds of twilight, known as astronomical twilight, nautical twilight and civil twilight. For each of these the evening twilight starts at sunset and morning twilight ends at sunrise. However, astronomical twilight ends (and begins) when the center of the Sun is 18 degrees below the horizon, nautical twilight ends (and begins) when the Sun is -12 degrees below the horizon and civil twilight ends (and begins) when the Sun is 6 degrees below the horizon. In general before morning and after evening civil twilight outdoor activities require artificial illumination. At the beginning and end of civil twilight the brightest stars are visible but the sea horizon is still clearly visible. At the beginning and end of nautical twilight the sea horizon is not clearly visible. The Astronomical Almanac reports the Zone Standard Time (ZST) of the end of evening astronomical twilight.

LMST (evening twilight)

The Astronomical Almanac reports the end of evening astronomical twilight in Local Mean Sidereal Time (LMST).

Morn Twilight

The Astronomical Almanac reports the Zone Standard Time (ZST) of the beginning of morning astronomical twilight.

LMST (morning twilight)

The Astronomical Almanac reports the beginning of morning astronomical twilight in Local Mean Sidereal Time (LMST).

Twilight (end)

The Astronomical Almanac reports the end of evening nautical twilight in Zone Standard Time (ZST).

Twilight (start)

The Astronomical Almanac reports the beginning of morning nautical twilight in Zone Standard Time (ZST).

Night Centre

The time of the center of the night is the moment the Sun reaches its lowest elevation below the horizon. The Astronomical Almanac reports the time of night center in Zone Standard Time (ZST).

Moon

The Astronomical Almanac reports a very simple description of the Moon's position at the time and date specified. It is either "UP" or "DOWN". Note that the times of moonrise or moonset are reported only if they occur when the Sun is below the horizon.

Age

The Moon's age is defined as the number of days elapsed since the most recent moment at which the selenographic colongitude of the Sun was 270 degrees, i.e. since the last New Moon. Note that the Moon's age is calculated for the moment of local midnight on the date specified rather than for the specified time. Note also that the age of the Moon is only calculated if it is above the horizon at the moment of local midnight, otherwise the age is reported as "Unknown".

Fraction

The Astronomical Almanac reports the fraction of illuminated area of the Moon or, in other words, its phase. For example at New Moon the fraction will be 0.0, at Half Moon it will be 0.5 and at Full Moon it will be 1.0. Note that the Moon's illuminated fraction is calculated for the moment of local midnight on the date specified rather than for the specified time. Note also that the Moon fraction is only calculated if it is above the horizon at the moment of local midnight, otherwise the fraction is reported as "Unknown".

Brightness

Astronomers usually need to know, at a particular time and date for a specific location, whether the sky will be dark enough for them to see very faint objects. Since it is mostly the Moon which floods the night sky with light and so hides faint objects, this involves a calculation of where the Moon is in the sky and what fraction of it is illuminated. The Astronomical Almanac reports a very simple description of the brightness of the sky for the specified date, time and location. It is either "BRIGHT" or "DARK".

Moon RA (right ascension)

The Astronomical Almanac reports the right ascension (RA) of the Moon for the date and time specified.

Moon Dec (declination)

The Astronomical Almanac reports the declination (Dec) of the Moon for the date and time specified.

Moonrise

Moonrise is defined as the time at which the apparent upper limb of the Moon reaches the astronomical horizon as the Moon's elevation is increasing. The Astronomical Almanac reports the moonrise time for the date and time given in Zone Standard Time (ZST). Since moonrise is only really important to astronomers if the Moon's light is likely to interfere with observations the Astronomical Almanac only reports moonrise if it occurs when the Sun is below the horizon. In this case the moonrise is reported as "daytime".

Moonset

Moonset is defined as the time at which the apparent upper limb of the Moon reaches the astronomical horizon as the Moon's elevation is decreasing. The Astronomical Almanac reports the moonset time for the date and time given in Zone Standard Time (ZST). As for moonrise the Astronomical Almanac only reports moonset if it occurs when the Sun is below the horizon. In this case the moonset is reported as "daytime".

Planetary Positions

The Astronomical Almanac reports a description of the positions of the Sun and Moon and all the nine planets of the solar system for the date, time and location specified. This description includes the right ascension, declination, hour angle, sec z (air mass), the elevation and azimuth. The planetary positions reported by the Astronomical Almanac are not very precise. For the inferior planets (those closer to the Sun than Earth) the accuracy is usuually less than 1 arcminute. For Mars the accuracy is a few arc minutes, for Jupiter about 0.1 degrees and for Saturn, Uranus and Neptune a few tenths of a degree. Pluto's positional accuracy is greater than this and can be seriously in error for dates far from 1992.