Astronomy Definitions

The "nested hierarchy" paradigm of envisioning our place in the universe takes that address as far out as we can take it. Here are the steps:

Planet Earth, a ball of rock and metal on whose surface all "geography" (including countries) exists. Earth's diameter is roughly 8,000 miles.

The Earth-Moon system, consisting of the Earth and the Moon, is easy to visualize as seen from somewhere else in space. The Moon is about 1/4 the Earth's diameter and is about 30 times the Earth's diameter away.

The Solar System, which contains the Earth-Moon system and many other planets and their moons, consists of the Sun (a star) and all things in orbit around it, including but not limited to:

Eight major planets (Pluto isn't considered "major" any more),

Dozens of satellites (moons),

Hundreds of thousands of asteroids, and

Uncountable numbers of smaller bits, including comet nuclei.

The scale of the solar system is such that if the Sun (in actuality about 800,000 miles in diameter) were represented by a soccer ball in the middle of the Planetarium, then the Earth would be an apple seed just outside the Planetarium building and the most distant "major" planet, Neptune, would be about the size of a marble at the corner of Stevens Creek and Stelling at the NE corner of campus. On this scale, the nearest other star is about 10,000 miles away! The distance unit that is most commonly used to express distances within our solar system - between planets, say - is the "astronomical unit", defined to be the average distance between Earth and the Sun, or about 93 million miles.

The Milky Way Galaxy contains the Sun (and its collection of planets) and an unimaginably large number of other stars: roughly 400 billion of them. 400 billion is about the number of seconds in 10,000 years, so don't bother asking why we haven't counted. Its extent is so vast that distances between its member stars are generally expressed in light years. One light year is defined to be the distance...