This page provides a synthetic image of a selected solar system object that approximates the telescopic view of the object as seen from the Earth at the specified date and time. Simply fill in the form below and click on the "Make Image" button at the end of the form. The solar system objects that you can choose are Mercury, Venus, the Moon, Mars, Jupiter, and the four Galilean satellites of Jupiter. An image can be produced for any date and time from year 1700 through year 2030 for any of the available objects. Specify Universal Time on the form.

It takes 5-20 seconds of CPU time (on our computer) to produce the image, depending on how busy it is and which object you select. The image is a grayscale JPEG (.jpg) of 20-40 kbytes, so bandwidth is usually not the major factor for wait time.

Be sure to check Notes on the Image, located after the form.

Notes on the Image:

The apparent disk of the object is shown within a square image of total size 700 x 700 pixels. The field in which the disk appears is oriented with north up and east to the left. These are directions on the celestial sphere: north is toward the north celestial pole of date, and east is parallel to the celestial equator of date, in the direction of increasing right ascension. Thus the orientation shown will not in general relate to the local horizon. That is, on the sky, "north" is usually not the same as "up".

The size of the disk shown will depend on the distance of the selected object from the Earth at the date and time specified. As much as possible, each disk is shown within a field of fixed angular size on the sky (for example, the field is 75 arcseconds wide for Venus); however, the field's angular size is reduced when necessary so that the disk width is never less than half the field width. The apparent equatorial diameter of the disk in arcseconds is listed below the disk.

The planetographic longitude and latitude of the sub-Earth point (at the center of the disk) and the sub-solar point (on the line between the center of the object and the Sun) are listed below the disk, as is the phase. The phase is the fraction of the area of the disk that appears illuminated by the Sun. The longitude system is that used by the International Astronomical Union (IAU) Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites. This system runs opposite the direction of rotation from 0 to 360 degrees; that is, for direct (prograde) rotation, longitudes are positive in the westward direction on the surface of the body. This is the system that is used in the Astronomical Almanac for the physical ephemerides of the planets. For the Moon and the Galilean satellites, all of which rotate synchronously with their orbital motion, longitude 0 is the meridian that on average points toward the planet they orbit. Note that westward on the surface of the body is generally toward the east on the geocentric celestial sphere.

The Moon's libration in longitude (= selenographic longitude of the Earth) is conventionally expressed in a different system from that used here. To obtain the usual libration in longitude, reverse the sign of the sub-Earth longitude given here and, if necessary, add 360 degrees to reduce the resulting angle to the range 0 to 10 degrees. The usual libration in latitude is the same as the sub-Earth latitude given here.

No surface features are shown for Mercury, since it is only partially mapped. Venus is covered with opaque clouds without visible structure, and you have the option of viewing either the cloud tops (a shaded featureless disk, the normal telescopic view) or the surface as revealed by radar mapping (without shading). The rotation of Jupiter used here is that of System II, which applies to the visible clouds outside of the equatorial region; the Great Red Spot is indicated at longitude 93 degrees. Other spots and cloud features at specific longitudes are temporary, and the Red Spot's longitude seems to be slowly increasing with respect to System II. The shadows of the Galilean satellites on Jupiter's disk are not shown (although this is a possible future enhancement).

To help in visualizing the orientation of the object selected, dotted lines of longitude and latitude are shown on the surface in black, every 30 degrees, beginning at 0 degrees longitude and latitude.

"Earthshine" is shown for the Moon. Here it is proportional to the phase of the Earth as seen from the Moon, which is the inverse of the phase of the Moon as seen from the Earth.

The orientation of the surface of the object is computed using the constants and formulas from the "Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2000" (Seidelmann et al. 2002, Celestial Mechanics and Dynamical Astronomy, Vol. 82, pp. 83-110). The Earth-Sun-object geometry is computed from the data in the Jet Propulsion Laboratory DE405/LE405 planetary and lunar ephemeris. Light-time is accounted for. These calculations are thus consistent with those used for the preparation of the Astronomical Almanac for years 2003 and later (except for minor differences for the Moon). The maps of the Moon, Mars, and the Galilean satellites are those of the U.S. Geological Survey. The map of Jupiter is a mosaic produced by the NASA Cassini imaging team.

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