At first it might not seem like the study of Earth would be an important part of astronomy. However, Earth is part of our solar system neighborhood, and many things happening on Earth are directly related to other objects in the solar system.
For example, auroras, tides, and seasons are all caused by Earth's interaction with the Sun or Moon. Comet and asteroid impacts change the surface of the Earth and can influence the evolution of life. Even topics in geology, such as volcanism, tectonics, and the study of rocks and minerals are useful in astronomy because they help us interpret what we see on other planets.
For instance, we know that on Earth the mineral hematite is often formed in hot-spring type environments, and so when Mars Global Surveyor detected hematite on the surface of Mars, it was evidence that there might once have been liquid water there.
Comparing Earth to other solar system objects can also teach us a lot about Earth's history. The early history of Earth is a good example of how this can work. Because Earth's rocks are recycled by plate tectonics, we have very few rocks dating back to Earth's beginning.
The oldest Earth rocks are about 3.8 billion years old, although some small, very old grains found in sandstone have been dated at 4.1 billion years. The solar system is believed to be about 4.6 billion years old. Therefore, there is little evidence left on Earth about the very early solar system. Some asteroids and comets, however, have not been altered much since their formation, and studying them can provide information about what the solar system was like when Earth was first forming.
Looking at old surfaces of larger objects, like our Moon, can also help us discern Earth's distant past. Studying craters on the Moon has shown us that our part of the solar system went through a period of "heavy bombardment" when large numbers of asteroids or comets impacted planets, moons, and each other. If the Moon was impacted many times, the Earth probably was too, even though the craters from this event have been destroyed by erosion and tectonics.
Many people find Earth facts and phenomena especially interesting because they relate to things we see every day