Topocentric Configuration of Major Solar System Bodies
This data service provides tables of low precision
topocentric
position
data for the Sun, Moon, and major planets at specified time(s). It is designed
to provide "quick look" information that should be useful for tasks such as
planning an observing session or pointing a telescope at one of the objects.
Data will be provided for the years 1800 through 2050.
Use the buttons under Location to find coordinates of cities or towns in the U.S. or its territories,
or to convert between Degrees-Minutes-Seconds (DMS) and Decimal Degrees.
Be sure to read the
Notes section
for additional details on the data.
Notes
Output Table
The
right ascension
(R.A.) and
declination
(Dec.) columns provide the topocentric apparent equatorial coordinates of each object.
Right ascension is rounded to the nearest tenth of a minute of time, and the declination
is rounded to the nearest arcminute. The distance (Dist.) column gives the true
distance from the observer to the object, rounded to the nearest 10-3 of an
astronomical unit (au) for the Sun and the major planets, and to the nearest kilometer (km)
for the Moon. (Note that 10-3 au is about 150,000 km, or 23.5 Earth radii.)
Apparent horizon coordinates, referred to the observer's location, are tabulated in the
zenith distance
(Z.D.) and
azimuth
(Az.) columns. The horizon
coordinates are tabulated to the nearest degree (atmospheric refraction is not taken
into account). The
elongation
(Elong.)
column gives the angular separation between the Sun and the object to the nearest degree.
The direction of the object with respect to the Sun is given by the letters N, S, E, or W
(north, south, east, or west) immediately preceding the angular measure. The diameter
(Diam.) column lists the equatorial diameter of the object's apparent disk (fully illuminated),
to the nearest tenth of an arcsecond.
The magnitude (Mag.) column provides the visual magnitude (not corrected
for zenith angle) for each major planet, rounded to the nearest tenth of a
magnitude. The magnitudes of the Moon and Sun are not listed. When Mercury and Venus
are near conjunction with the Sun, their computed magnitudes are not realistic and
are not tabulated (i.e., dashes appear in the table in place of a numerical value).
For Saturn, the magnitude includes the contribution due to the rings. For the Moon,
the percentage of the disk that is illuminated is given in place of the magnitude.
The apparent visual magnitudes for the planets may differ slightly from those
published in
The Astronomical Almanac.
Except for Mercury and Venus, the expressions used here for the major planets are from Harris (1961)
[1].
The expressions for the magnitudes of Mercury and Venus are based on the parameters given
in Hilton (2005) [2].
Through its 2004 edition, The Astronomical Almanac used the work of Harris
(1961) for all the planets. In 2005 and 2006, it adopted values for Mercury and Venus
from Hilton (2003) [3].
From 2007 through 2020, The Astronomical Almanac has followed Hilton (2005)
for these two planets. Beginning with the 2021 edition, The Astronomical Almanac uses
the expressions found in Mallama & Hilton (2018)
[4]
for the apparent magnitudes of all the planets.
Eclipses and Transits
This data service also checks for the occurrence of eclipses of the Sun and Moon
and transits of Mercury and Venus across the Sun. If one of these phenomena occurs,
a message will appear directly below the main body of the table. The message will
appear only if the phenomenon occurs when the objects involved are above the observer's
horizon. It is important to understand that these messages describe the instantaneous
status of the phenomenon at the time and location of interest. For example, a message
such as "Sun in partial eclipse" means that at the time and location indicated, the Sun
is partially obscured by the Moon, not that the eclipse necessarily is classified as a
partial eclipse. A configuration table computed for the same date but some other time or
location might indicate total or annular obscuration.
UT1
UT1 is a form of Universal Time
affected by irregularities in the Earth's rotation, and is the modern version of mean
solar time on the Greenwich meridian.
Height
The user can specify the height of the observer, which can range from the surface of
the Earth to a maximum of 10,999 m (the top of the troposphere). Although the observer's
height is used in determining the positions, the effect is minimal and not usually apparent
to the precision displayed here. However, due to the Moon's proximity, the effect on lunar
distance is more noticeable.
- Harris, D. L. 1961, "Photometry and Colorimetry of Planets and Satellites," in
Planets and Satellites, ed. G. P. Kuiper & B. A. Middlehurst (Chicago:
University of Chicago Press) pp. 272-342.
- Hilton, J. 2005,
"Improving
the Visual Magnitudes of the Planets in The Astronomical Almanac. I. Mercury and
Venus"
The Astronomical Journal, Vol. 129, pp. 2902-2906.
- Hilton, J. 2003, "Updating the Magnitudes of the Planets in The Astronomical
Almanac," USNO/AA Tech Note 2003-04 (Washington: USNO)
- Mallama, A. & Hilton, J.L. 2018,
Computing Apparent Planetary Magnitudes for The Astronomical Almanac
,
Astronomy and Computing, Vol. 25, pp. 10-24.
