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.
Be sure to read the Notes section (on this page beyond the two forms) for definitions and additional details on the data.
Form A - U.S. Cities or Towns
Form B - Locations Worldwide
Notes on the data:
The right ascension (R.A.) and declination (Dec.) columns provide the topocentric apparent equatorial coordinates of each object, rounded to the nearest tenth of a minute of time in right ascension and to the nearest arcminute in declination. The distance (Dist.) column gives the true distance from the observer to the object, rounded to the nearest 10-3 of an astronomical unit (A.U.) 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 of 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.
Expressions for the apparent visual magnitudes of the major planets (except Mercury and Venus) are from Harris (1961) . The expressions for the magnitudes of Mercury and Venus are based on the parameters given in Hilton (2005) ; these values may differ from those given in editions of the The Astronomical Almanac published before 2007. Through its 2004 edition, The Astronomical Almanac used the work of Harris (1961) for Mercury and Venus as well. In 2005 and 2006, The Astronomical Almanac adopted values for Mercury and Venus from Hilton (2003) . Since the 2007 edition, the The Astronomical Almanac has followed Hilton (2005) for these two 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.
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.