## Mars24 Frequently Asked Questions

(Updated Feb. 26, 2006)

There are just a few questions and answers so far.

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

Do the following steps:

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

If you watch the sunclock for a minute, 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 most personal telescopes display things upside down. Additionally, there may be another 10 or 20° of rotation to the image depending on the relative orientation of Mars and Earth's poles.

Remember also that Mars24 sunclock is based on a stock image with enhanced contrast, and since it uses a single such image it does 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 quite like the Mars24 display.

#### Why is there a difference of 41 minutes between MER-A Spirit mission time and Local Mean Solar Time (LMST) at the Spirit landing site?

The MER-A planners at the NASA Jet Propulsion Laboratory specified that (1) the mission clock should use mean Mars time units as the clock ticks, but (2) the mission clock should be roughly aligned with local true solar time at the middle of the originally planned mission duration of 90 sols. This second requirement, in other words, means that on about the 45th sol of the Spirit mission, noon on the mission clock was within 30 seconds of true solar noon at longitude 184.5215°W.

(As of this writing, Spirit was in its 765th sol of operation. The planned duration has been greatly exceeded!)

The difference between mission time and LMST at the Spirit site is quite large because when Spirit landed on Mars, the planet was at the point in its orbit where the difference between Local True Solar Time (LTST) and LMST — what the technical notes called the Equation of Time — was near its maximum value. So although on the day of landing there was a 10-Mars minute difference between the mission clock and LTST, the difference between the mission clock and LMST was over 41 Mars minutes.

#### 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 NASA JPL for the Mars Exploration Rovers 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 at JPL did not decide to simplify matters and specify that the mission clock scheme which would make the difference exactly 12 Mars-hours.

#### 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 point, this is displayed as -0.2°.