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.
The constellations you see depend on your latitude, and not your longitude. Latitude measures your north-south position. Longitude measures your east-west position.
Why do constellations depend on latitude and time?
The movement of the sky changes with your latitude. In the Northern Hemisphere, if you were looking at the North Star, it would appear that the sky is turning counterclockwise around the north pole. However, in the Southern Hemisphere, looking towards the south pole, the sky would appear to turn in the opposite direction - clockwise - around the south pole.
Time is a factor since constellations visible at a particular time of night change as we orbit the Sun. Visibility is affected by the position of the Earth relative to the Sun and stars. Therefore, the time of year plays a significant part in what constellations you can see in the night sky.
People often wonder why stars appear to vanish during the daytime and come out at night, but the stars are always present. You do not see them during the day because their dim lights are overwhelmed by the bright daylight sky. You can see bright stars during the daytime using a telescope. You can also see the stars during the daytime if you travel through space as an astronaut, because you are above the Earth's atmosphere where there is no air to diffuse sunlight. As an astronaut, you see the Sun as a bright disk against a dark sky filled with stars.