Mars24 Sunclock — Time on Mars

Frequently Asked Questions

There are just a few questions and answers.

Why is there a difference between the Mars InSight / MSL Curiosity / Mars Phoenix mission time and Local Mean Solar Time (LMST) at the landing site?

When each of these projects was planned, a mission clock was specified based on the Local Mean Solar Time (LMST) at the longitude targeted for landing. In each of these cases the lander touched down at a different longitude, but the definition of the mission clock did not change.

For Mars InSight, a landing site at longitude at 135.97°E was planned. Sol 0 was defined as commencing at LMST midnight for that longitude the night before landing was to occur. However, InSight touched down about 0.35° west of that longitude, and a location of 135.625°E is used in the Mars24 landmarks list. The difference in Local Mean Solar Time for these two longitudes is about 84 seconds.

(In future versions of Mars24, the difference for Mars InSight is expected to change. At this writing, there has been no official announcement of a more exact set of coordinates for the InSight landing site. However, once the longitude is better known, the difference in LMST should remain within the range of 75 to 95 seconds.)

For MSL Curiosity, a landing site at longitude at 137.42°E was targeted. Again, Sol 0 would commence at LMST midnight for that location, the night before landing was to occur. The landing site coordinates were refined through course corrections were made while MSL was in-flight to Mars, and the rover touched down somewhat "long" of the final target coordinates. The landing site turned out to be at 137.4416°E. The difference in LMST between these two longitudes is about five seconds.

For Mars Phoenix, the planned landing was at longitude 233.35°E. Sol 0 would commence at LMST midnight for that longitude, the night before landing. However, the choice of landing site was not yet final. Two months before landing, the Phoenix team decided to shift the landing to 233.975°E. But Phoenix touched down long, about 0.25° east at 234.2485°E. The end result was a difference of about three and a half minutes between the mission clock and LMST at the landing site.

As the discrepancy between the mission clock and LMST is not important to mission operations, but making changes to mission timekeeping tools (even before landing occurred) could require more effort than any benefit that might be gained, planners for each of these missions saw no need to re-define the mission clock.

Why is there a large difference between MER Spirit and Opportunity mission time and Local Mean Solar Time (LMST) at the landing site?

During mission planning, the Mars Exploration Rover (MER) teams specified that the mission clock for each MER lander should 1) use mean Mars time units as the clock ticks, but 2) be roughly aligned with Local True Solar Time (LTST) at the middle of the originally planned mission duration of 90 sols (i.e., on about sol 45 of thir missions). This second requirement has two effects, first due to the large difference (roughly 50 minute) between LMST and LTST on Mars in January 2004, and then subtracting an offset of either 10 minutes or 13 minutes from that difference. Tere would be some aditional smaller adjustment due to the landers touching somewhere other than the exact longitude planned. The end result is that both MER rovers used mission clocks roughly 40 minutes behind LMST at their landing sites.

The difference between LMST and LTST was so large because when Spirit landed on Mars, the planet was at the point in its orbit where the difference between LMST and LTST — what the technical notes call the Equation of Time — was near its maximum value. The difference was somewhat less but still significant when Opportunity landed.

(Note: Spirit operated for over 2200 sols, and as of June 2018, Opportunity had exceeded 5100 sols of operation. The planned mission durations were greatly exceeded!)

Why do the lander mission times for Spirit and Opportunity differ by 12 hours plus/minus a minute and 10 seconds?

As noted above, the mission times defined by the planners for the Mars Exploration Rover project were defined so that at about the middle of each rover's planned mission, the individual mission clocks would approximately sync up with Local True Solar Time at the lander sites. This means that the mission times are calculated separately and have no direct relation to each other.

Unfortunately, although the mission times for the two landers turned out to be very close to being 12 Mars hours apart — which is roughly what one expects; the two landers are almost on opposite sides of the planet — mission planners did not decide to simplify matters and define mission clocks that would make the difference exactly 12 Mars-hours.

How do I set Mars24 to show me what Mars looks like from Earth?

Perform the following steps in the settings window:

  • Select the sunclock display and set it to use the "Orthographic" map projection.
  • Set the "center of map" to use the landmark pop-up menu.
  • Select "Sub-Earth point" in the landmark pop-up menu.

If you watch the sunclock for few minutes, you will occasionally see a slight shift in the graphics as Mars rotates within the view.

If you're trying to compare the resulting image to what you might see through a telescope, keep in mind that the display does not account for the time it takes light to get from Mars to Earth. Depending on where the two planets are in their orbits, this can take 3-20 minutes, and during that time Mars will rotate from one to five degrees.

Remember also that Mars24 sunclock uses stock images with enhanced and/or false color and enhanced contrast. The "realistic" images included in Mars24 do not include any seasonal effects that alter the appearance of Mars, such as dust storms and growth/contraction of the polar caps. So even if you had a super high resolution telescope like the Hubble, Mars probably would not look like the Mars24 display.

Why does the table on the panorama display say that the Sun had an elevation of -0.2° at sunrise and sunset? Shouldn't it be 0.0°?

On Mars, just like on Earth, the Sun is not so far away that it is a pinpoint light source. Depending on where Mars is in its orbit, the Sun has an apparent radius between 0.16° and 0.19°. Mars24 defines sunrise and sunset as occurring when the edge of the Sun crosses the horizon, which is to say, when the center of the Sun is at some elevation between -0.16° and -0.19°. Rounded off to a single digit after the decimal character, this is displayed as -0.2°.

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