In simplest terms, an orbit is the path of a planet around the Sun, or of a satellite around its parent body. The Moon orbiting Earth is an example. But what causes the orbit to have one size and not another? It turns out that all of the properties of an orbit are the result of two physical laws: the law of energy conservation and the law of conservation of angular momentum.

Vectors are mathematical objects. They are too complicated to be described by mere numbers, but like numbers, vectors have a size. However, they also have a direction. As such, they can be used to mathematically describe certain physical items that also have a size and a direction.

Velocity is an example of such a vector quantity; it has both a size or amount (speed) and a direction. What do we mean when we say that a velocity has a direction? A velocity is always forward, back, left, right, up, down or some combination of these. This makes it a perfect candidate for a vector description.

In this article we outline the basic properties of vectors, provide several examples of physical quantities that are described by them and also demonstrate how they are used to describe the world.

Astronomers often speak of dark matter and dark energy, terms that are confusing for the public and not very descriptive. In brief, both are invisible substances, meaning that they do not interact with any type of light, from radio to gamma-rays, and their existence is only inferred because of the effect they have on the motion of material that we can see. Here we will look at the evidence for dark matter, as well as the ideas scientist have postulated for what it might be. The dark energy will be the subject of another posting.