Assignments:

Unfinished Assignment Study Questions for Lesson 4

Lesson Objectives:

- The sky and constellations
- Points of reference
- Angular size and distance
- Latitude and time

The patterns in the sky that we see are called constellations. A constellation is a region of the sky with well-defined borders. Bright stars help us identify constellations.

Our galaxy, the Milky Way, is shaped like a thin pancake that has a bulge in the middle. We are located on Earth about halfway out from the center of this pancake. From this perspective, we see the stars and interstellar clouds that make up the Milky Way in the night sky.

The local sky is the sky as seen from where you happen to be standing. This appears to be a dome or a hemisphere.

The horizon is the boundary between earth and sky. The zenith is the point directly overhead on the horizon.
The meridian is an imaginary half-circle stretching from the horizon due south though the zenith to the horizon due north. We use these points of reference for the local sky and we can pinpoint the location of an object in the local sky by looking at its altitude and direction above or along the horizon. Altitude here refers to the angle above the horizon. Direction is the direction in relationship to the horizon.

For example, you can have a person that points to a star located in the SE direction at 60 degrees altitude.

Angular size (also called angular diameter) is the angle that an object appears to span in your field of view.
The farther away an object, the smaller its angular size. For example, the angular size of the Sun and Moon are roughly 1/2 a degree, but this does not reflect true size. Angular size is affected by distance. The Sun has the same angular size as the Moon even though it is roughly 400 times bigger than the Moon because it is about 400 times as far away!

The angular distance, or the distance between a pair of objects in the sky, is the angle between them. It is a rough estimate based on our view of the local sky.

For greater precision, we use arcminutes and arcseconds. Arc minutes are the minutes that comprise a degree, or in other words, one-sixtieth of a degree.

Arc seconds are the seconds that comprise an arcminute, so an arc second would be one-sixtieth of an arc minute.

For example, we would read the angular distance above as 37 degrees, 27 arcminutes, 15 arcseconds.