Above what line of latitude or longitude do geostationary satellites orbit? (A) The Tropic of Cancer, (B) the Tropic of Capricorn, (C) the equator, (D) the prime meridian, (E) 60 degrees longitude.
Let’s begin by sketching a diagram for each option, showing Earth and each proposed orbital path. Before we get into specifics about geostationary orbit, we can narrow down our options just by thinking about some basic properties of any orbit. Recall that gravity is an attractive force that holds any orbital system together and that it acts along a straight line that connects objects’ centers of mass. We can draw a diagram to better visualize this. Imagine this is Earth and this is a satellite orbiting Earth. In order for the satellite to maintain a nice circular orbit at a constant speed, there must be a force pulling the satellite toward the center of orbit at all times. That force is gravity, and we know it’s pulling the satellite directly toward Earth’s center of mass.
Now, let’s look at answer options (A) and (B). They suggest orbits that remain above the lines of latitude 23.5 degrees north and 23.5 degrees south, respectively. It’s actually impossible for either of these orbits to be naturally sustained. Remember that because of gravity, the satellite’s center of orbit must correspond to the Earth’s center of mass. Because of this, for option (A) to be viable, Earth’s center of mass would have to be somewhere up here closer to the North Pole than the South Pole. Likewise, option (B) suggests that the Earth’s center of mass is somewhere down here closer to the South Pole than the North Pole. We know that both of these are false. In reality, Earth’s center of mass aligns with the center of the planet, which must therefore also be the center of a satellite’s circular orbital path.
Answer choices (A) and (B) are incorrect, but the rest of the options do agree with this observation. (C), (D), and (E) show orbits that are centered around the center of Earth. These three orbits are physically possible. But are they geostationary? Recall that a geostationary satellite stays above a fixed point on Earth’s surface. It can do this by completing one orbit in the same amount of time it takes Earth to complete one rotation. Recall the Earth’s axis of rotation runs through the North and South Poles, and Earth rotates from west to east. Looking at option (C), so long as the satellite moves west to east as well, its orbit can be synchronized with the planet’s rotation so that the satellite appears to be stationary from an observer here on Earth.
Option (C) looks correct. But what about (D) and (E)? Both of these are considered polar orbits since they pass directly over the North and South Poles. Their only difference is that for (D) the satellite also passes over the prime meridian, which marks zero degrees longitude, and the orbit shown in (E) passes over 60 degrees longitude. In both cases, a satellite would be moving perpendicular to Earth’s direction of rotation. Therefore, it wouldn’t even matter if the satellites did have an orbital period of 24 hours because they’d be constantly moving with respect to the surface.
The satellites would always be changing latitude and thus could not be geostationary. We should eliminate (D) and (E). Answer choice (C) is correct, and thus we have seen why geostationary satellites must orbit above the equator.