Apparent Disk of Solar System Object
Astronomical Applications Dept. Astronomical Applications Dept.
 
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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 2100 for any of the available objects. Specify Universal Time on the form.

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

             

    Universal Time (UT1)

   

(NOTE: It can take as long as 10-15 seconds to render a new image. Please be patient.)

Notes on the Image:

The apparent disk of the object is shown within a square image of total size 1024 x 1024 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.

Below the image the sub-Earth coordinates, sub-solar coordinates, the illumination phase, and the apparent equatorial diameter of the body are listed.

The sub-Earth point is defined as where a line connecting the center of the Earth and the center of the body intersects the body's surface. Practically speaking, this can be visualized as the surface location at the center of the body's apparent disk. Similarly, the sub-solar point is defined as where a line connecting the center of the Sun and the body crosses its surface. The locations of these points are reported in planetographic latitude and longitude coordinate systems appropriate for the body being viewed. For all objects, latitude is analogous to Earth's latitude; that is, the number of degrees north or south of the body's equator. For planets and the Galilean moons, the longitude system is that used by the International Astronomical Union (IAU) Working Group on Cartographic Coordinates and Rotational Elements. This system of longitude increases in the direction opposite the body's 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 (note that westward on the surface of the body is generally toward the east on the geocentric celestial sphere). The Moon is a special case since it is in synchronous rotation about the Earth; for an observer on the Earth the Moon does not appear to rotate smoothly but rather through its libration oscillate a few degrees east and west. Due to this various coordinate systems for the Moon have been utilized. We report the sub-Earth and sub-solar points for the Moon in selenographic coordinates, which has longitude defined as 0 degrees for the location that on average is facing the Earth and increasing in the opposite direction as the Moon's terminator moves (from left to right for an Earthbound observer). Similarly, the Galilean moons are in synchronous rotation about Jupiter and have their 0 degree longitude points defined as the location facing Jupiter. In this fashion the coordinates reported are in systems consistent with those used in the Astronomical Almanac for the physical ephemerides of all objects.

Next to the sub-Earth and sub-solar coordinates the illumination phase is reported. The phase is simply the fraction of the area of the apparent disk that appears illuminated by the Sun. Finally, the apparent equatorial diameter of the disk is listed (the difference between equatorial and polar diameters is significant only for Jupiter). This value is displayed in arcseconds for all objects except the Moon. The Moon has its diameter reported in units of arcminutes/arcseconds due to its large apparent size.

The orientation of the surface of the object is computed using the constants and formulas from the "Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009" (Archinal et al. 2011, Celestial Mechanics and Dynamical Astronomy, Vol. 109, no. 2, February, pp. 101-135). 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).

Object-specific notes:

Mercury

No surface features are shown for Mercury, since it is only partially mapped.

Venus

Venus is covered with opaque clouds without visible structure. Selecting Venus' default view results in a featureless disk similar to Mercury. However, Venus has been extensively studied using radar imagery to penetrate the thick clouds. Selecting this option will show a radar map of Venus in the correct oriention relative to Earth but with no illumination shading (no visual illustration of its current phase).

Mars

The map of Mars is courtesy of USGS Astrogeology Science Center.

Jupiter

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). The map of Jupiter is a mosaic produced by the NASA Cassini imaging team. The maps of the Galilean satellites are courtesy of the USGS Astrogeology Science Center.

The Moon

"Earthshine" is shown for the Moon and is the reason why for phases other than full the entire disk is visible, albeit at a lower surface brightness than the portion facing the Sun. 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 map of the Moon is courtesy of the USGS Astrogeology Science Center.

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