Why did Ptolemy have to introduce multiple circles of motion for the planets instead of a single, simple circle to represent the planet’s motion around the Earth?

Astronomers call the side of the Moon that always faces away from the Earth the dark side of the Moon.

Frozen water exists on the lunar surface.

What type of eclipse is this and what is the phase of the moon?

What is the disk-shaped belt of icy bodies believed to extend from about 30 AU to 50 AU from the sun?

The Hubble constant is about 70 km/s/Mpc70 km/s/Mpc, which means that a galaxy traveling at 2800 km/s away from the Milky Way is about 40 Mpc away.  What would the velocity of the Milky Way be as seen from such a galaxy?

Match the components of the Milky Way Galaxy to their definitions.

The globular clusters in our galaxy form a ‘swarm’ that is centered on

Which type of galaxy always has an active nucleus?

How do astronomers explain the fact that all galaxies outside our Local Group show a redshift?  The universe is