Mars24 Clock User's Guide

(Updated Nov. 30, 2004)

Mars24 is a cross-platform Java application which displays a Mars "sunclock", a graphical representation of Mars showing the current day- and nightsides of Mars, along with numerical readouts of the time in 24-hour format. Other displays include a plot showing the relative orbital positions of the four terrestrial planets and a panorama showing the solar path as seen from a given location on Mars.

Mars24 cannot accurately determine the time on Mars unless your computer's time, time zone and date are set correctly.

Please read the accompanying Technical Notes on Mars Solar Time for a more detailed discussion about the meaning of the various display values and for definition of Mars time units.

When first launched, Mars24 displays three windows: a settings window for controlling display options, a small display window showing the time on Mars and on Earth, and a large display window which can show either 1) a sunclock map of Mars, 2) an orbital positions plot, or 3) a local panorama plot for a location on Mars. If you close a window, you can make it visible again using the appropriate command in the Window menu. The Window menu also allows you to display the help pages.

On most computers, closing all windows will cause Mars24 to quit. If you are using a Macintosh, however, you must use the Quit command in the application menu to exit the program.

Popup Menus

When viewing either the time display window or the graphic display window, you may right-click (Windows, Linux) or control-click (Mac OS X) on the window content to bring up a popup menu (aka, a contextual menu) which offers some choices on changing the display. Among these options is the ability to resize the window to one of several pre-defined sizes.

Saving Settings

When you quit Mars24, it saves the settings of the various controls to a preferences file. The next time you use the program, it will open that preferences file and initialize the controls using the prefs it finds.

If you would like to save several different configurations of the controls, you can use the Open and Save Settings commands in Mars24's File menu to do so. Saved prefs files must be given the extension .m24. A few sample prefs files are included with Mars24. (Macintosh users: if you doubleclick on a .m24 prefs file, Mars24 should automatically open it.)

Settings Window

This window is divided into two tabbed panels: one tab allows you to specify how to set the display time for both Earth and Mars, and the other to specify properties of the large graphical display. We'll start by taking a look at the time tab.

The Earth time settings in the first tabbed panel gives you three choices for specifying what time on Earth should be used in making all the various calculations that Mars24 must perform, both in determining the Mars time and generating the various displays.

The first (default) Earth time choice is to use the current time, i.e., now, whatever "now" happens to be.

The second choice is to add some offset to the current time on Earth. For example, if you need information about the time on Mars exactly 100 Earth hours from now, you would select this option and enter "100" in the hours field. Note that when you select this option, the clocks in the time display keep on ticking, but are just offset from the current time by whatever amount have entered.

To find out the time on Mars that corresponds to a specific Earth time, you would select the third choice and enter a time of day in the first input field and a date in the second field. The time and date must be in UTC (Coordinated Universal Time), which is in everyday usage effectively the same as Greenwich Mean Time. Please note that the Java routine which we use to parse the date entry is somewhat finicky about the format of the entered date. It understands common date notation such as "July 20, 1976" but not ISO date format such as "1976-07-20".

Also in the Earth time settings is a checkbox which allows you to specify how you would like Earth date shown in the display windows. Normally it is shown in ISO format, i.e., "YYYY-MM-DD", but if you opt to show the date as day-of-year (a scheme that planetary mission controllers often use) then the format changes to "YYYY-DDD".

The Mars time settings effectively gives you two choices, to either display the local time for some particular place on Mars or to display the mission time for one of the current lander projects. If you choose the first option, you also need to select a time format. When Mars24 is first launched, the default is to display the local mean solar time on the Mars prime meridian.

For example, if you would like to display the time at a Olympus Mons, you would click the "local time" radio button and enter into the location fields longitude 133.10°W and latitude 18.60°N.1 (Actually, the N-S latitude doesn't matter when calculating the time, so you could just leave the latitude field at 0°N.) The adjacent format menu gives you the option of displaying the time in either local mean solar time (LMST), local true solar time (LTST), or local mean zonal time (MTC+N). Zonal time is marked by a suffix indicating the time zone, e.g., "MTC-2" identifies the time zone two hours behind the prime meridian. (See the "Notes about Mars Time" help page for further information about these formats.).

The alternative Mars time setting is to display the mission time for one or both of the current lander missions, i.e., MER-A Spirit and MER-B Opportunity. The mission time will be shown as a time of day and a Sol value, the latter being the count of the number of Martian days since landing. "Sol 1" is defined as the Martian solar day on which the lander touched (or will touch) down, and the day before landing is "Sol -1". (Obviously, then, there is no "Sol 0".) The time of day displayed starts ticking at local midnight immediately prior to landing, not at the actual time of landing. If you are displaying the time for an Earth date prior to landing, the time of day will instead read either "T.B.L." (to be launched) or "In Flight".

The mission time for the two Mars Exploration Rovers ticks away using the mean values for the Martian time units. However, the MER mission times are not the same as local mean solar time for the landing sites. Instead they are offset from local mean solar time by several minutes (and not by the same amount each) so that at the middle of the planned mission duration (three months each), the mission time and local true solar time would just about match up. (For more discussion of this, see the FAQ.)

We'll come back to discussing the plot settings tab below when we discuss the graphic display window.

Time Display Window

This window is divided into four quarters, grouped two above and two below. Let's take a look at the bottom two first.

The most prominent items in the bottom two quarters of the time display are the Earth time readouts, at left in UTC and at right in the user's local time zone, according to the choice selected in the Earth time settings. Below each of these is the corresponding date. Remember, both UTC and local time (and consequently the time on Mars) will not be correct unless your computer's time and time zone have been set correctly.

Below the Earth UTC time and date is the corresponding Modified Julian Day (MJD). The Julian Day (JD) is the number of days that have passed since noon on Jan. 1, 4713 BCE. This count is a very useful value in astronomy and is often used to indicate the dates of astronomical events and observations, especially those which predate use of the modern Gregorian calendar. However, in the 20th and 21st centuries, the Julian Day is a large and unwieldy number, so we subtract 2,400,000.5 to obtain the easier-to-use Modified Julian Day, the number of days since midnight (UTC) starting Nov. 17, 1858.

Below the user's local time and date is the one-way light distance (OWLT) between Earth and Mars, i.e., the amount of time a photon would take to traverse the distance between the two planets. Depending on the current relative orbital positions of the two planets, this value ranges between 3 and 22 minutes.

In the upper half of the time display window are the Mars time read-outs. If you have selected the default choice in the settings, then just as for Earth the top left will show the "standard Mars time" and the top right a local time. The latter is determined by the location you specified in the Mars time settings.

The "standard Mars time" is given using the designation "MTC", or Coordinated Mars Time. In analogy to Earth's UTC, this is the local mean solar time at Mars' prime meridian, which is defined by the location of the crater Airy-0.

As we noted above, the local Mars Time readout is given for a specific longitude and latitude and uses one of three different time formats which you may choose from in the settings. The lon-lat chosen is shown immediately below the local time.

The upper half of the time display window also contains a few other numeric readouts. At left, below the MTC, are three items giving some information about the Mars "date". At present, there is no recognized Mars calendar so these data are shown in an alternative fashion. First, in analogy to the Julian Day on Earth, there is a readout for MSD, or Mars Sol Date, i.e., a count of the Mars days — sols — that have elapsed since humans began to make useful observations of the surface features of Mars in the late 19th century. MSD 0.0 is approximately the same moment in time as MJD 5521.5 (noon on Dec. 29, 1873) on Earth.

Below the MSD is a readout of the Areocentric Solar Longitude, denoted (LS), the angle that Mars has reached in its orbit relative to vernal (spring) equinox in the northern hemisphere. Below the LS value is an indication of the corresponding Martian. The abbreviation "NH" or "SH" which appears here stands for "northern hemisphere" or "southern hemisphere", as appropriate.

On the right side of the Mars time info, below the local time readout, are entries for the solar elevation and solar azimuth. For the location on Mars at which the local time is being calculated, these two values indicate the current position of the Sun, the elevation indicating its position above the horizon and the azimuth its position relative to due north.

The layout of the upper half of the time display, i.e., the sections displaying the time on Mars, will re-arrange itself if you have used the time settings to specify a mission time display. The only change in what is actually displayed is that the local time and solar position are replaced by the appropriate lander mission time and that the solar position is omitted.

Graphic Display Window and Settings

The largest window displayed by Mars24 is the graphic display, which initially shows a sunclock of Mars. The other two types of plots which may shown in this window are an orbital position plot and a local panorama plot. Use the menu at the top of the plot tab in the settings window to choose which plot to display.

Sunclock Display

The sunclock settings allow you to specify what map projection should be used to render the map, where that map should be centered, and what special markings should be drawn on the map.

At start, Mars24 defaults to using an "orthographic" projection, which renders a fairly realistic view of how Mars looks from space. The projection menu provides nine other choices, the two most popular of which are probably the equirectangular and Mollweide projections.

Next, you may choose the location on which the map projection should be centered. The controls allow you to select from a variety of landmarks, including a number of lander and geographic sites, or to specify a particular longitude and latitude. Alternatively, you can shift-click on the map itself, and the map will re-center at location where you clicked. (Note: Only the orthographic projection can be centered at a latitude off the equator.)

You can also specify "how dark" you would like the nightside of Mars shaded, with 100% meaning completely black and 0% no shading at all. A value of about 70% works well, but depending on your computer monitor you might find it helps to raise or lower this value a bit. The next checkbox allows you to indicate whether a lon-lat grid, with 30° spacing, should be overlaid on the map.

The final set of options in the sunclock settings is a table of checkboxes indicating which, if any, of a set of locations should be marked on the map. The first two checkboxes are to mark the "Subsolar point" and the "Sub-Earth point", by a yellow circle and blue circle, respectively. The subsolar point is the location on Mars for which the Sun is directly overhead. Likewise, the sub-Earth point the location at which Earth is directly overhead, or for an observer on Earth looking at Mars, it is the spot directly in the middle of the hemisphere in view.

The next ten points of interest in the table are lander sites, in order, the Mars Exploration Rover mission sites, the three past successful landers (Viking 1 and 2 and Pathfinder), and the five failed landers2. Following the landers is a selection of notable surface features.

When examining the sunclock, please keep in mind that the base map used to render this plot comes from a composite of Mars Global Surveyor imagery acquired during northern hemisphere winter, when the north polar ice cap was relatively large and the solar polar ice cap relatively small compared to their average extents. During southern hemisphere winter, the south polar cap becomes significantly larger, with a layer of frost extending almost all the way to the edge of Hellas Planitia.

Orbital Positions Display

The second graphic display option is a plot of the orbital positions of the four terrestrial planets. This plot has no special options of its own in the prefs panel.

Mars's orbit is shown as a red ellipse and the planet's position is marked by a capital M, Earth is in light blue and marked by a capital E, Venus is in yellow and marked by a capital V, and Mars is in white and marked by a lower-case m. The orbit of Jupiter and the other planets beyond Mars are not shown as they are too big to fit into the display window.

The other small markings on the orbital ellipses indicate the locations of perihelion, marked with a p, and the northern hemisphere vernal (spring) equinox, marked ve. Perihelion is a planet's closest approach to the Sun; its aphelion, or farthest distance from the Sun, is indicated by an unlabeled tick mark on the far side of the orbital ellipse from perihihelion. Similarly, the start of the other seasons are indicated by unlabeled tick marks at 90° intervals from the vernal equinox mark.

You might find it interesting to use the Earth time prefs to specify an Earth time and date in the morning of Aug. 27, 2003, and then examine the orbital positions plot. You'll see that Earth and Mars lie almost on the same line from the Sun, with Mars at its perihelion and Earth about 45° from its aphelion. This was the Great Mars Opposition of 2003, when Earth and Mars were the closest they had been in about 59,000 years. The light distance between the two planets on that date was just 3 minutes and 6 seconds. (Note: astronomical opposition was actually on Aug. 28, but because Earth and Mars orbit in slightly different planes, closest approach was on Aug. 27.)

Local Panorama Display

The final graphic display is a panorama as might be seen from the location of Mars you have specified in the Mars time tab. The location of the Sun and Earth as seen from that location are marked on the panorama, the Sun by a yellow circle smaller blur circle. A grid is marked on the plot indicating the four cardinal directions.

Below the panorama is a table of numerical readouts which lists the times during the day of certain events related to the positions of the Sun and Earth. These include the time of highest ascent in the sky (for the Sun, that's true solar noon) and lowest descent (for the Sun, true midnight), plus the times of crossing the horizon (sunrise, sunset, Earthrise and Earthset).

If you specify that the location of the panorama is either the MER-A Spirit or MER-B Opportunity landing site, the plot will use a panorama photo taken by the appropriate lander as a backdrop. Otherwise it will use a solid Martian reddish color to represent the ground.

The first setting or the panorama display allows you to alter whether the panorama plot or the readout table is omitted from the display. This is useful if you want to draw the plot bigger or if you want to be able to see the entire readout table without scrolling.

The other settings for this display allow you to specify whether the Sun and Earth should be shown on the panorama plot as plain dots (yellow for the Sun, blue for Earth) or with their paths marked. The paths can be rendered as simple curves or as curves with tick marks. The ticks indicate where the Sun or Earth will be at intervals of one Mars-hour.

The determination of sunrise and sunset accounts for the fact that the Sun is not a pinpoint light source but has, on average, an apparent radius of 0.175° as seen from Mars. (It ranges from 0.193° at perihelion to 0.160° at aphelion.) Denoting sunrise as the time when the limb of the Sun reaches the horizon, then on average sunrise begins when the center of the Sun is 0.175° below the horizon.

Please note that the determination of times of sunrise, sunset, earthrise, and earthset does not adjust for any refraction of sunlight by Mars' atmosphere, nor is there any accounting for local topography at the location selected. Also note that the accuracy of the calculations of these event times will be improved if you specify correct planetographic latitude and longitude when you select the location for which you wish to see the panorama.


1. You can look up the coordinates of various features on Mars using the US Geological Survey Gazetteer of Planetary Nomenclature's Mars page. Be sure to reference the index which uses planetographic latitude with west longitude.

2. Following is the roster of lander missions and few notes about each. Again, all locations are in planetographic coordinates.

Active Landers (as of 2004-11-13):

Past Successful Landers:

MPF and VIK coordinates are per Folkner et al. (1977), with longitude adjustment due to redefinition of prime meridian in Seidelman et al. (2002).

Unsuccessful Landers:

The NASA Mars Scout lander called Phoenix is planned for touchdown in May 2008. It is expected to land in the Vastitas Borealis, in the vicinity of 120°W, 70°N.

The next likely rover mission is the Mars Science Laboratory, which would land in 2010. Its landing locations has not yet been determined.

The current version of Mars24 may be found at

Mars24 was written by Dr. Robert B. Schmunk. If you wish to be notified when new versions of Mars24 are released or if you would like to report a bug, please contact:

Robert B. Schmunk
NASA Goddard Institute for Space Studies
2880 Broadway
New York, NY 10025 USA

Technical questions related to the definition of Mars Solar Time and the mathematical algorithms adopted for its calculation by Mars24 may be directed to:

Michael D. Allison
NASA Goddard Institute for Space Studies
2880 Broadway
New York, NY 10025 USA