Mars Viewer Help

This form enables you to generate a Postscript file showing the appearance of the Martian system at a specified time. All bodies are rendered with terminators and shadows as appropriate. Selected background stars can also be included.

In the diagram, Mars and the moons are modeled as triaxial ellipsoids, and are drawn with latitude and longitude contours at 15 degree intervals. Illuminated regions are indicated with black lines; unilluminated regions and terminators are shown as light gray. Penumbral shadows are not indicated.

Optionally, the hypothetical rings of Mars are also shown. Their appearances are based on the model of

In this model, each of Mars' moons generates a cloud of dust. The Phobos ring is modeled as equatorial but 350 km thick and shifted toward the Sun by one Martian radius. The Deimos ring is 5300 km thick and shifted away from the Sun by two Martian radii. It is also tilted out of the equatorial plane toward the ecliptic by 15 degrees. See the above reference for the explanations of these surprising properties.

Each ring is shown by a pair of curves, one indicating each of its vertical limits. The rings are shown in black if illuminated and in gray if not or if the opposite side is illuminated. The Phobos ring is shown by solid lines; the Deimos ring by dashed lines.

The diagram is oriented with J2000 declination increasing upward and with right ascension increasing to the left. The frame has uniformly-spaced tick marks along each axis. The declination axis is labeled in degrees, minutes and seconds; the right ascension axis is labeled in hours, minutes and seconds.

Each diagram includes a caption that summarizes the key parameters used to generate it.

Change History

2.0 (November 9, 1999): Original Mars viewer on line. (The version number was chosen for consistency with other tools.)
2.1 (December 1, 2009): Updated the default ephemerides. To reduce confusion, we have removed the choice about what ephemeris to use.
2.8 (January 23, 2013): Renumbered for consistency among all Planet Viewer tools. Ephemeris updates.
2.9 (January 4, 2016): Ported to new server. Ephemeris updates.
2.10 (October 14, 2020): Ephemeris updates.

Observation Time

The observation time (UTC) can be entered in a variety of formats. For example, the following all parse to 0:01:02 UTC on July 4, 1976: If you want the gory details of how times are interpreted, click here.

Field of View

Enter the field of view of the drawing to be generated and select the appropriate units. Units can be seconds of arc (the default), Martian radii, or kilometers (projected at the distance to Mars).

Diagram Center

Three different methods of specifying the diagram center are supported. Click on the box to the left of the option you wish to use.

Body: The diagram will be centered on the location of the selected body.

Ring ansa: The diagram will be centered on the ansa of the selected ring. Specify the east or west ansa using the second box. Note that east is toward the left in the diagram.

J2000 RA and dec: The diagram will be centered on the specified pair of right ascension and declination coordinates. The first box is for the RA and the second box is for the dec. Enter up to three values in each box, separated by spaces. The values are interpreted as hours, minutes and seconds, respectively in the RA box and as degrees, minutes and seconds in the dec box. Any or all values can have fractional parts.


You may specify the point of view of the diagram. By default, the point of view is the center of the Earth.

Observatory: You may select from any viewpoint on the list.

Latitude & Longitude: If your desired observatory or location is not on the observatory list, you can enter its latitude, longitude and altitude in the three boxes provided. Latitudes and longitudes can each be specified by up to three values, interpreted as degrees, minutes and seconds. Longitudes can be specified either east or west.

If you wish to have an observatory added to the standard list, or to refine the coordinates of a listed observatory, email the necessary information to Mark Showalter.

Ring Selection

You can decide whether to include the hypothetical Martian rings in the diagram. Click on the box to the left to select your option. If selected, both rings are indicated by pairs of dashed lines above and below the ring plane to indicate these rings' physical thickness. The Phobos ring is shown with a solid line and the Deimos ring is shown with a dashed line.

See above for a discussion of the models used for each ring.

Background Objects

You have several independent options for including background objects in the diagrams. These options make it possible to render diagrams around the times of stellar occultations. Objects are marked by pluses and are labeled by name if the Moon & Star Labels option is activated.

Standard stars: Check the box to include any of a standard list of stars that happen to fall inside the field of view of the diagram. The Mars Viewer does not access a star catalog; instead, it only plots stars from a finite list. To view the current star list, click here. This list is updated periodically at the request of the users; if you would like to have a star added to the list, email the necessary information to Mark Showalter.

Additional star: In addition to or instead of the standard stars, you can specify one additional star to be included in the diagram. Check the box to the left and enter the star's RA, dec, and name in the three boxes. Enter up to three values in the RA and dec boxes, separated by spaces. The values are interpreted as hours, minutes and seconds, respectively in the RA box and as degrees, minutes and seconds in the dec box. Any or all values can have fractional parts.


Enter a title for the plot in this box. It will appear centered above the diagram.

Moon and Star Labels

Optionally, the diagram will be generated with the name of each moon and star written above and to the right of its center. You may select the size of these labels in points, where a point is 1/72 inches.

Moon Enlargement

The diagram is rendered to scale, which means that Mars' moons may be very hard to see. You may enter a minimum plotted size for moons in this box. If a nonzero value is used, the smallest moons will be easier to see. The size is in units of points, equal to 1/72 inches. A value of ~4 may be appropriate.

Blank Disks

If you activate this option, all latitude and longitude lines will be suppressed in the diagram. This produces a diagram that is suitable for amateur observers to use as a drawing blank.
Mars Viewer Form | Node Tools | Ring-Moon Systems Node Home

Last updated October 14, 2020.

Mark Showalter