Mars24 Sunclock — Time on Mars

User's Guide

Mars24 is a cross-platform Java application that 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 inner planets, a panorama showing the solar path as seen from a given location on Mars, and the Mars analemma.

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 definitions of Mars time units.

When first launched, Mars24 will displays two windows: a display of the time on Mars and on Earth and a display showing a sunclock map of Mars. This second display window can be changed to show an orbital positions plot, a panorama plot for a location on Mars, or the Mars analemma.

A window with controls for setting the time and display parameters can be invoked using menu commands.

Time Display

Mars24's first graphic display is a list of three or more clock readouts. It always shows two readouts for Earth time, one of them for UT (Universal Time, essentially the same as Greenwich Mean Time, GMT) and one for your local time zone, as determined from your computer's time settings. The third readout always shown is the time at Mars's prime meridian, or "Airy Mean Time" (AMT), an analogy to Greenwich Mean Time on Earth.

When Mars24 is first launched, it will also show the mission clocks for the two currently active lander missions: MSL Curiosity and InSight. Curiosity is the rover that touched down early August 2012, while InSight is the stationary lander that touched down in November 2018.

To add additional clock readouts to the time display, click on the "plus" button in the lower left of the time display window. A dialog will appear offering you a variety of choices for setting up the readout. You can select a lander mission clock, the time at a specified Martian landmark, or at a location specified by a pair of longitude-latitude coordinates. (Please note that Mars24 expects lon-lat coordinates to be entered with planetocentric, positive-east values).

For example, if you would like to display the time at a Olympus Mons, you would select a clock view type of "Named Landmark" and then select "Olympus Mons" in the landmark menu. Alternatively, you might specify a view type of a lon-lat location and manually enter the coordinates for Olympus Mons (see endnote 1).

If you specify a view type of either a named landmark or a lon-lat location, you also need to specify a clock "format". The choices are the location's Local Mean Solar Time (LMST), Local True Solar Time (LTST), or local mean zonal time (LMST, AMT+N). (See the "Notes about Mars Time" page for further information about these formats.).

The "date" shown below the location's time is the Ls value, a measurement in degrees of Mars's orbital position relative to when the vernal (spring) equinox occurs in its northern hemisphere.

If you choose to display a lander mission time, the readout will show a time of day and a mission date for one of the several current or past successful lander missions. How these mission clocks are defined has varied from mission to mission. In all but one case, the time of day has been specified as the Local Mean Solar Time (LMST) at the planned landing site or as some offset from that LMST. (See the "Notes about Mars Time" help page or the FAQ for further detail on the offset amounts.) The mission date is shown as a Sol number, a count of the number of Martian days since landing. For some landers, the Martian solar day on which they landed was defined as "Sol 1" while for others it was instead defined as "Sol 0".

If you add a clock readout to the time display and later decide you no longer need to see that readout, then click on the readout in the display, and then click on the "minus" button in the lower left of the window.

At the lower right of the time display is another button that shows a clock face. Click on this button to bring up the Settings window and show the time controls.

Setting the Earth Time

This "Mars24 Settings" window is divided into three tabbed panels: one tab allows you to specify how to set the Earth time used in all calculations, the second to specify properties of the large graphical display, and the third is for miscellaneous options.

The Earth time settings panel gives you five 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 displays.

The first Earth time choice is to use the current time, in other words, now, whenever "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 you 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 UT (Universal Time), which is in everyday usage effectively the same as Greenwich Mean Time.

The fourth and fifth options for choosing what Earth time should be displayed involve specification of the Julian Date (JD). This 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. Because the Julian Date for modern dates is a relatively large and unwieldy number, the fifth input choice is to use the Modified Julian Date, the number of days since midnight UT starting Nov. 17, 1858.

Next in the time settings is a checkbox that allows you to specify how you would like the Earth date formatted in the time 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".

Also in the time settings are two pop-up menus that allow you to specify the units in which the distance between Earth and Mars is displayed. One menu is for displaying the one-way light time (OWLT), the amount of time that it takes light to travel between the planets. The other menu is for selecting between simple units of length, either kilometers or astronomical units.

Graphic Display Window and Settings

Mars24's large display window initially shows a sunclock of Mars. The other three types of plots that may be shown in this window are of orbital positions, a local panorama, and the Mars analemma. Use the menu at the top of the plot tab in the settings window to choose which of these to display.

Sunclock Display

The sunclock is simply a map showing the dayside and nightside of Mars, Initially this map uses an "equirectangular" map projection. It is likely you will want a view that shows Mars as a globe, in which case you would set the sunclock's map projection menu to show an "orthographic" map. The Mollweide projection is also a popular choice as it is equal-area.

The source map control allows you to choose between viewing several different map images in the sunclock display. The two most "realistic" are the MOC-MOLA-NGS map, created by the Mars Global Surveyor project for National Geographic magazine, and the VIS-MDIM map, which is based on Viking orbiter imagery. Other choices include a false color topographic image and two black-and-white topographic images. You might notice that some of these maps are located in a directory called "sourcemaps" in the Mars24 distribution. If you have other full global equirectangular maps of Mars, centered on the prime meridian, you can drop them into this directory and they will become available for Mars24 to use the next time you launch the program.

(When using the NGS or the Viking map, please keep in mind that these images were acquired at particular times of the Martian year. The polar caps that they show would not match what would be seen at other times of the year.

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 three lines of controls affect whether and how a longitude-latitude grid and the datum line should be drawn on the sunclock map. Although it looks a bit like a coastline, the datum line is really just the line of average altitude and has nothing to do with seas or oceans that might have existed on Mars in the past.

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 dozen points of interest in the table are lander sites (see endnote 2), first all the planned or successful missions followed by those which did not work out. Following the landers in the table is a selection of notable surface features. The coordinates of all the fixed locations are taken from the "marslandmarks.xml" file which is included in the Mars24 distribution and which Mars24 reads in when it first starts. You can change the color with which any of these sites and landmarks is marked on the sunclock by either right-clinking (Windows, Linux) or control-clicking (macOS) on the location in the table, and then selecting a color from the pop-up menu that appears.

Orbital Positions Display

The second graphic display option is a plot of the orbital positions of the four inner planets: Mercury, Venus, Earth and Mars. Depending on how you have sized the plot window, part of Jupiter's orbit might also be visible.

Mars's orbit is shown as a red ellipse, and the planet's position is marked by a ♂ symbol. Earth is in light blue and marked by a ⊕ symbol, Venus is in yellow and marked by a ♀, Mercury is in white and marked by a ☿, and Jupiter, if visible, is in orange and marked by a ♃. The orbits of the other planets beyond Mars are not shown because they are much too big to fit into the display.

The small tickmarks on the orbital ellipses indicate the locations of perihelion, labeled with a small p, and the northern hemisphere vernal (spring) equinox, labeled ve. Perihelion is a planet's closest approach to the Sun; its aphelion, or farthest distance from the Sun, is indicated by a smaller 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 tick, going counterclockwise. Depending on control settings, the aphelion and additional equinox/solstice locations may also be labeled.

You might find it interesting to use the Earth time settings 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 time between the two planets on that date was just 3 minutes and 6 seconds.

Local Panorama Display

The panorama display shows the locations of the Sun and Earth in the sky, as seen from a location on Mars as specified in the panorama plot controls. The Sun is marked by a yellow circle and Earth by smaller blue circle. A grid is marked on the plot indicating the four cardinal directions.

Below the panorama is a table of numerical readouts that lists the times during the day of certain events related to the positions of the Sun and Earth. These include the time of its zenith or highest ascent in the sky (for the Sun, that is the same as true solar noon), and also its nadir or lowest descent (for the Sun, true solar 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, MER-B Opportunity or Mars Phoenix landing site, the plot will use a panorama photo taken by the appropriate lander as a backdrop. Otherwise it will use a solid reddish color to represent the Martian ground.

The next setting in the panorama controls allow you to choose what time format (LMST, LTST, or LMZT) should be shown for the Mars times in the panorama table.

After that is a control that can be used to alter whether the panorama graphic 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 the panorama 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 about 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's 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.

Analemma Display

This diagram plots the Equation of Time (the difference between difference between the True Solar Time (TST) and the Mean Solar Time (MST)) and the solar declination as functions of the areocentric longitude, Ls. Or in other words, for a given position in Mars orbit, what are 1) the discrepancy between true solar noon and mean solar noon, and 2) the Sun's angle relative to the equatorial plane. The first of these is measured along the plot's x-axis and the second along the y-axis. The units of the x-axis are Mars minutes.

For example, on the first sol of Mars's northern hemisphere spring (Ls=0), you'll see that the solar declination is zero (exactly as it should be for an equinox) and that the discrepancy between true and mean solar time is a bit more than 41 minutes.

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 initialize settings and controls to the saved values.


1. You can look up coordinates for numerous surface features on Mars using the US Geological Survey's Gazetteer of Planetary Nomenclature's Mars page.

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