MICA Physical Ephemerides Calculations

The physical ephemeris of a solar system body refers to its aspect as seen from the Earth: its apparent magnitude, the angular size of its disk, its apparent degree of illumination, the orientation of its pole, and the positions of its sub-solar and sub-Earth points. This information divides into illumination data and rotation data. Illumination data depends on a model of the body's reflectivity as a function of angle of illumination, while rotation data depends on a model of the body's rotation; both depend on the Earth-Sun-body geometry. Illumination information includes the object's apparent phase, magnitude, and the angular dimensions of its disk. Rotation information includes the instantaneous positions of the sub-solar and sub-Earth points on the object's surface and the apparent position angle of the object's axis of rotation.

In MICA's physical ephemeris tabulations, longitudes and latitudes are planetographic, and position angles are measured on the sky eastward from true north (the direction to the true, Celestial Ephemeris Pole of date). The illumination and rotation data, respectively, are the same as those found on the left-hand and right-hand pages of the 'Ephemeris for Physical Observations' section in section E of The Astronomical Almanac. All of MICA's physical ephemerides are available using either a geocentric or topocentric origin. Except for the Moon, and Venus and Mars when near the Earth, the geocentric physical ephemeris of an object is indistinguishable from the topocentric physical ephemeris. Rotation data is available for the Sun, Moon and major planets. Illumination data is available for the Moon and major planets, but not the Sun.

The MICA physical ephemerides of the planets have been calculated using the basic physical data (directions of the north poles of rotation, the prime meridians, and the size and shapes of the major planets) contained in Seidelmann (2002). Expressions for the apparent visual magnitudes of the major planets (except Mercury and Venus) are from Harris (1961). Values for V(1,0), the magnitude of a planet as seen from 1 AU and at a phase angle of 0, are given on page E88 of The Astronomical Almanac. The MICA Version 2.0 expressions for the magnitudes of Mercury and Venus are based on the parameters given in Hilton (2003) and are the same as used in the 2005 and 2006 editions of The Astronomical Almanac. The MICA 2.0 Mercury and Venus magnitudes differ slightly from the 2004 edition (and earlier editions) of The Astronomical Almanac, which used the earlier Harris (1961) expressions. A useful discussion on the calculation of physical ephemerides is also contained in Hilton (1992).