Satellite Overpass Answers

After reaching a consesus within your group,
let's see if your answers are like the ones provided below.

Question 1: Which direction does the satellite appear to be moving with respect to the earth's surface?

Terra appears to be moving from the northeast towards the southwest during hours of daylight, or in other words a descending orbit.
Aqua and Aura appear to be moving from the southeast towards the northwest, or ascending with respect to the equator.
Remember, you must use the "DDD" values for peak angle to get these results; if you use the "NNN" values you will be just opposite these apparent directions since the earth will be facing away from the sun at these times each day; thus the satellites will on the opposite side of the earth from the sun.

Question 2: Which direction is the earth turning with respect to the sun?

You should have gotten this answer correct. Since the sun rises in the east and sets in the west, the earth spins from west towards east. Some prefer to say counter-clockwise when viewed from above the north pole. Actually, it spins (rotates) at the rate of 15 degrees every hour, or 360 degrees in 24 hours, each day.

If you really want to get specific about, "What our day length is?"; look further into this subject to reveal just how long an Earth day really is. Perhaps a little "Extra Credit" could be negotiated with your teacher?

Question 3: If you have access to a globe of the earth you may also ask, "What kind of orbit must the satellite be moving in order to follow this path?".

Actually, these satellites as well as the LANDSAT satellites follow a "near" polar orbit, that is they revolve approximately around the poles of the earth. In reality, many such remote sensing satellites follow a sun syncronous orbit, or in other words, they cross the equator near our longitude at about the same time each day. Aqua and Aura actually have an "orbit inclination" of about 98 degrees, thus most of the "apparent" slant in their path is due to this factor rather than the Earth's rotation during overpass times.

Possibly a trip to the dictionary is in order. Look up the word "ECLIPTIC" and relate it to the concept of inclination. Now that is worth extra credit for sure!

Question 4: After plotting all three, what similarities do you notice about them?

Aqua and Aura are part of the "A-Train", a NASA plan to fly several earth orbiting satellites in a formation to gather a more robust data set than any one satellite alone can provide. Aqua, Aura, CloudSat (when launched) and a few other satellites will orbit earth in this configuration to study our atmosphere.

Aqua and Aura are in ascending polar orbits while Terra follows a descending polar orbit. Details are available at the links provided at the bottom of the previos page.

Finally, just how far away are these satellites as they pass overhead. Well, by using some trigonometry and assuming an altitude of 694 kilometers for the satellite orbit (and making estimates or straight line distances of the more complex factors involved) the following approximate horizontal distances are estimated:


Peak Angle (from the horizon) in degrees

Distance away from your site (horizontal)


531 km


446 km


347 km


237 km


180 km


121 km


60 km


15 km


1 km

Thanks for taking the time to plot these satellites and learn about their orbits. Maybe you can find other such information to develop into more elaborate investigations involving science, math, and geography.

Last Revised: 10 Sep 2004
by Allan W. Geery