Mars24 Sunclock — Time on Mars
Frequently Asked Questions
1. Why is there a difference of some seconds/minutes between the 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 probe touched down at a different longitude, but the definition of the mission clock did not change.
For InSight, a landing site was selected at longitude 135.97°E. However, the lander touched down about 0.35° west at 135.6234°E. The difference in Local Mean Solar Time between these two longitudes is about 83 seconds.
For MSL Curiosity, a landing site at longitude 137.42°E was targeted. The landing site coordinates were altered slightly by course corrections made while MSL was in-flight to Mars. The rover then 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 site used in specifying a mission clock was at longitude 233.35°E. However, while the probe in flight to Mars and about two months before landing, the Mars Phoenix team decide to shift the landing to 233.975°E. But Mars 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.
For each of these Mars landers, the discrepancy between the mission clock and LMST was not important to mission operations. Moreover, making changes to mission timekeeping tools would require more effort than any benefit that might be gained. Thus, the science and engineering teams saw no need to re-define the original mission clock.
2. Why is there a large difference between MER Spirit and Opportunity mission time and Local Mean Solar Time (LMST) at their landing sites?
During mission planning, the Mars Exploration Rover project specified that the mission clock for each MER rover 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 their respective missions). This second requirement has two effects, first due to the large difference (roughly 50 minutes) between LMST and LTST on Mars in January 2004, and then subtracting an offset of either 10 minutes or 13 minutes from that difference. There would be some additional smaller adjustment due to the rovers touching down somewhere other than the exact longitude planned. The end result is that both 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.
3. 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, their separate mission clocks would approximately sync up with Local True Solar Time at the lander sites. This means that the mission times were calculated separately and had 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 in which the time of sol would make the difference exactly 12 Mars-hours.
4. 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.
- Select an appropriate input in the source map pop-up menu.
If you watch the sunclock for a few minutes, you will see a slight shift in the graphics as Mars rotates within the view.
If you are 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 also 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 would still not look like the Mars24 display.
5. 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°.