Our Moon changes its appearance from day to day. It seems dim at first crescent, and steadily increases in brightness from day to day as its phase increases, until it reaches its Full Moon phase, under which you can see your shadow very well.

As the Moon changes phases, we on Earth can see an increasing percentage of the illuminated portion of the Moon. At New Moon, the Moon's entire far side is lit by the Sun, so we cannot see the Moon at all. Several days after New Moon, we can see a very small percentage of the illuminated portion of the Moon (first crescent). As the days progress, we will see increasing percentages of the Moon's illuminated side, giving us a larger light source every day as more light is being reflected toward us. The result is a brighter Moon.

Satellites also exhibit phases. If a satellite is between the observer and the Sun, the satellite is being lit from the opposite side, and is therefore invisible. As the satellite orbits the Earth, we might be able to see increasingly larger portions of its illuminated side, thereby making it more visible to us.

The phase angle quantifies the phase of the satellite and is defined as the angle at the satellite subtended by the observer and the Sun. The phase angle can range from 0 (lowest) to 180 degrees (highest). A low phase angle will generally indicate that the satellite will be brighter. A high phase angle will indicate that the satellite will mainly be lit from behind, thereby making it more difficult to detect.

A phase angle of 180 degrees indicates that the satellite is in-between the observer and the Sun and therefore back lit. A phase angle of 90 degrees indicates that 50 percent of the satellite's illuminated side can be seen. This is much like a half moon. A phase angle of 0 degrees means that the satellite is on the opposite side of the observer from the Sun but this does not mean 100 percent lit!

A satellite does not have its own light source. It can be seen because sunlight is being reflected off it. When a satellite is directly opposite the Earth from the Sun, the Earth blocks the Sun's light to the satellite. This is called an eclipse. A satellite that is being eclipsed by the Earth will become invisible to all visual observers on the Earth!

When you observe a satellite passing through your sky, make a note how its brightness appears to change as it travels. You might notice a large change in brightness from when you first see it to when you last see it. In some cases, a satellite can go from a 180 degree phase angle to a 0 degree phase angle in as little as 15 minutes!