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Explanation of the Solar System Ephemeris

General Comments

A Solar System Ephemeris is a table of the positions of the objects in the solar system: the Sun, Moon and Planets. This version of the table first repeats information from the input form fields: UTC Adjustment, Date, Time, Longitude, and Latitude. Then it gives the Julian Day, the Local Sidereal Time, the Moon Phase. Finally, it gives the positions of Sun, Planets and the Moon in a variety of astronomical coordinate systems: J2000.0 equatorial right ascension and declination, Horizon azimuth and altitude, and Ecliptical solar elongation and constellation, along with the angular size and percent illumination.

The ephemeris uses angular distances for a few of the columns. You can estimate angular distances using your hand. When your arm is fully extended, the width of your thumb is about 2°, the width of your fist is about 10°, and the distance between the tip of your thumb and the tip of your index finger (thumb and finger fully separated) is about 15°.


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Sample Solar System Ephemeris

Universal Time (UTC): Saturday  2000-07-15 03:00:00
UTC Adjustment (Time - UTC):              -05:00
                                -------------------
Date, Time:           Friday    2000-07-14 22:00:00

Longitude: West 87.91278° =  87° 54' 46"
Latitude: North 42.17861° =  42° 10' 43"

Julian Day: 2451740.62500000 ( +65.7s = Dynamical time)
Local Sidereal Time: 16h 00.7m
Moon Phase: Full -16°

         ------J2000.0------  ----Horizon---  ---Ecliptical--  Anglr
Object       RA        Dec    Azimuth    Alt   Elong  Constel  Size  Illum
-------  ---------  --------  --------  ----  ------  -------  -----  ----
Sun      07h 39.0m  +21° 29'  138° NW   -13°          Gem      31.5'
Mercury  06h 44.8m  +18° 25'  148° NNW  -22°   12° W  Gem      10.5"    8%
Venus    08h 19.0m  +20° 51'  130° NW    -9°    9° E  Cnc       9.9"   99%
Mars     07h 22.7m  +23° 02'  142° NW   -14°    4° W  Gem       3.6"  100%
Jupiter  04h 04.4m  +19° 55'  190° N    -26°   50° W  Tau      35.2"
Saturn   03h 45.1m  +17° 43'  196° NNE  -28°   55° W  Tau      17.1"
Uranus   21h 30.1m  -15° 33'  294° ESE   +2°  153° W  Cap       3.7"
Neptune  20h 31.3m  -18° 41'  306° SE   +10°  167° W  Cap       2.3"
Pluto    16h 42.0m  -10° 58'    0° S    +37°  138° E  Sco-Oph   0.1"
Moon     18h 32.1m  -22° 26'  333° SSE  +21°  164° E  Sgr      29.6'   98%

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Universal Time (UTC)

Coordinated Universal Time (UTC) is the basis for civil time. It is the standard time at the prime meridian (longitude zero, the meridian that passes through Greenwich, England), with no change for daylight saving time. In actual practice, this is the time kept by atomic clocks around the world.


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UTC Adjustment (Time - UTC)

The UTC Adjustment (Time - UTC) is the difference between the time at the observation site and UTC. It is shown as entered on the input form. Click here for an explanation of this input form field.


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Date, Time

The Date, Time is the date and time at the observation site. It is shown as entered on the input form. The mathematical connection between Universal Time (UTC), the UTC Adjustment (Time - UTC), and the Date, Time is also shown.


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Longitude and Latitude

Longitude and Latitude are the coordinates of a location on the Earth. Longitude is the angular distance west or east of the prime meridian (longitude zero, the meridian that passes through Greenwich, England); Latitude is the angular distance north or south of the equator. They are shown as entered on the input form. Click here for an explanation of these input form fields.


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Julian Day

The Julian Day is a count of the number of days (and fractions) since noon on January 1, 4713 BC. It is used in astronomy as a daily time scale to avoid the complications of civil calendars which have changed many times over the years. The Calendar FAQ and the sci.astro FAQ, Part 3 have more information about calendars and the Julian Day.


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Dynamical Time

In the late 1930's, astronomers noticed that errors in the computed positions of the Sun, Planets and the Moon could be explained by slow variations in the Earth's rotation. Dynamical Time was eventually developed as a uniform astronomical time scale, taking into account both the variations in the Earth's rotation and relativistic effects. The actual difference between UTC and Dynamical Time cannot be known precisely in advance. It must be measured when it occurs, so the program estimates the difference in order to use Dynamical Time as the basis for all the calculations. The sci.astro FAQ, Part 3 has more information about the problems of defining a uniform astronomical time scale.


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Local Sidereal Time

Local Sidereal Time is formally defined as the right ascension of the local meridian. If your observing site is in the Northern Hemisphere, any object whose right ascension is equal to the Local Sidereal Time is directly south (directly north for a Southern Hemisphere observing site). Sidereal time is measured in hours and minutes, with 24 hours for a complete sidereal day.

In the Sample Solar System Ephemeris, the Local Sidereal Time is 16h 00.7m. Pluto, whose right ascension is 16h 42.0m, will cross the local meridian in 41.3m of sidereal time. Therefore, it must already be in the south sky, as the azimuth indicates.


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Moon Phase

One complete lunar cycle consists of four phases:

The four lunar phases.
Phase Illumination Position of Moon, Earth, Sun
New Moon 0% Moon between Earth and Sun
First Quarter 50% Moon 90° E of Sun
Full Moon 100% Earth between Moon and Sun
Last Quarter 50% Moon 90° W of Sun

The orbit of the Moon

Each phase is actually just a moment in time since the Moon is constantly moving with respect to the Sun. The program shows the Moon Phase and the angular distance from that phase moment. Negative angular distances indicate that the Moon has not yet reached the actual phase moment, positive angular distances indicate that the Moon has already passed it. Since there are four phases, each one takes up 90° (and thus the angular distance from the phase moment ranges from -45° to +45°). The phase moments occur at 0° from the Sun (New Moon), 90° east of the Sun (First Quarter), 180° from the Sun (Full Moon), and 90° west of the Sun (Last Quarter).

In the Sample Solar System Ephemeris, the Moon Phase is Full -16°, indicating that it will be Full (180° from the Sun) in 16°. Thus, its solar elongation is 180° - 16° = 164° E, as the ephemeris indicates.


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J2000.0 Equatorial: Right Ascension and Declination

The Equatorial coordinate system is a spherical coordinate system which uses the Earth's equator as the reference plane, and the Earth's axis as the polar axis. Just as Longitude and Latitude describe a point on the Earth, so do right ascension and declination describe a point in the sky. Right ascension, (RA) is the angular distance measured in hours and minutes eastward along the equatorial plane from the vernal point (the point where the ecliptic crosses the equator, where the Sun goes from below the equator to above it, located in the constellation Pisces). Declination (Decl) is the angular distance measured in degrees and arcminutes from the equatorial plane, positive north of it, negative south of it. You can find more information about astronomical coordinate systems at the SEDS organization web site.

Since the Earth's axial tilt and the Earth's orbit is constantly changing, the reference time for the coordinate system (the epoch) must be specified. This program uses J2000.0, the beginning of the Julian year 2000, Julian Day 2451545.0, which is the epoch of modern star charts. Thus, these coordinates are useful when placing the objects on a J2000.0 star chart.


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Horizon: Azimuth and Altitude

The Horizon coordinate system is a spherical coordinate system which uses the observer's horizon as the reference plane, and the observer's zenith (the point directly overhead) as the polar axis. Azimuth is the angular distance measured in degrees westward along the horizon from the south. Altitude is the angular distance measured in degrees from the horizon plane, positive above it, negative below it. This program does not take atmospheric refraction into account; objects near the horizon plane will appear slightly higher than they really are. You can find more information about astronomical coordinate systems at the SEDS organization web site.


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Ecliptical: Solar Elongation and Constellation

The Ecliptical coordinate system is a spherical coordinate system which uses the Earth's orbital plane (the ecliptic) as the reference plane. The object's angular distance above or below the ecliptic is ignored here.

The solar elongation (Elong) is the angular distance of the object from the Sun, as seen from the observer's site, along the ecliptic plane, measured in degrees and direction. Objects which are west of the Sun are morning objects; they rise earlier than the Sun. Objects which are east of the Sun are evening objects; they set after the Sun.

The solar elongation can also tell you the approximate rise or set time for the object. First find the rise or set time for the Sun. This can be determined from the newspaper, for example, and other sources (or simply by observing). Then divide the elongation by 15 to get the approximate number of hours earlier (if the direction is West) or later (if the direction is East) that it will occur for the object. For the date entered in the Sample Solar System Ephemeris, Saturn is 55° West of the Sun. It will rise approximately 3 hours and 40 minutes (= 55 / 15 = 3.666 hours) earlier than the Sun, and Jupiter will rise approximately 20 minutes (= (55 - 50) / 15 = .333 hours) later. Of the two bright stars in the morning sky that day, the one further east would be Jupiter (since it rises later than Saturn). Objects with elongations of about 12° or less are too close to the Sun to be seen. Mercury, Venus and Mars are all lost in the solar glare on the date entered.

The ecliptical constellation (Constel) is the abbreviation for the constellation whose boundaries include the ecliptical longitude of the object. The constellations, their abbreviations and their English names are:

The ecliptic constellations.
Constellation Abbr English name
Pisces Psc Fishes
Aries Ari Ram
Taurus Tau Bull
Gemini Gem Twins
Cancer Cnc Crab
Leo Leo Lion
Virgo Vir Virgin
Libra Lib Scales or Beam Balance
Scorpius Sco Scorpion
Ophiuchus Sco-Oph Serpent Holder
Sagittarius Sgr Archer
Capricornus Cap Sea Goat
Aquarius Aqr Water CArrier

Traditionally, each of the original twelve ecliptical constellations was assumed to include exactly 30° of ecliptical longitude, so that the sky was divided equally among them. This program insteads uses the actual boundaries as set by the International Astronomical Union (IAU), and so this information can be used to locate objects visually once you know the constellations. Note that Ophiuchus is included in the table because the ecliptic actually passes through this constellation just north of Scorpius. An object in Ophiuchus will use an abbreviation of Sco-Oph to show the connection of Ophiuchus and Scorpius. For more information about the ecliptic (zodiacal) constellations, see my tutorial: Learn the Zodiac (Ecliptic Constellations)!

You can find more information about astronomical coordinate systems at the SEDS organization web site.


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Angular Size

Each of the objects in the ephemeris can be seen as a disc in a telescope with the proper magnification. This column gives the size in arcseconds (") for the planets, and in arcminutes (') for the Sun and the Moon. (60" = 1', 60' = 1°.)


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Percent Illumination

The Moon, Mercury, Venus and Mars can be less than 100% illuminated. The Percent Illumination is an indication of the phase of the object, as described in Moon Phase above. The Moon, Mercury and Venus can have values from 0% to 100% illumination. Since Mars' orbit is outside the Earth's, Mars can only have values between 84% and 100%. (Jupiter can actually be less than fully illuminated, but the illumination can never be less than 99%).


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Last modified: Wednesday, 16-Sep-2015 22:12:16 EDT