Uploaded October 2021 | Updated September 2026, 2 hours ago
In the last lesson, we talked about the night sky, so everything you see when you look up at night, but how do we really take what we see and turn it into meaning? What information do all of those sights give us?
Today, we’ll be talking about our local sky. How we measure and map our place in the heavens.
In the last video, I told you about the celestial sphere, the useful tool we have for visualizing the sky as a map. But what do you actually see when you go outside? You don’t see a sphere. You see a dome, and depending on where you live, that “dome” has different stars visible at different times. Let’s go over some key terms:
The boundary between the earth and thes sky is called the horizon,
The point directly overhead is the zenith.
The meridian is an imaginary half circle stretching from the horizon due south, through the zenith, to the horizon due north.
Our lack of depth perception on the celestial sphere means we have no way to judge the true sizes or separations of the objects we see in the sky. However, we can describe the angular sizes or separations of objects without knowing how far away they are. The angular size of an object is the angle it appears to span in your field of view.
For example, the angular sizes of the Sun and Moon are each about 1/2°,
Note that angular size does not by itself tell us an object’s true size, because angular size also depends on distance.
Check this out, The Sun is about 400 times as large in diameter as the Moon, but it has the same angular size in our sky because it is also about 400 times as far away.
You can actually estimate angular size with your hand. Stick out one finger at arms length and that’s about 1 degree wide. And you can think of 1 degree as 1 slice of a pie with 360 slices in it. And your fist, is approximately 10 degrees. To go a little deeper into the measurement of the sky, since some stars appear so small, we need to break things down a little more.
We do this by dividing 1 degree into 60 arcminutes (symbolized by ‘) , and 1 arcminute into 60 arc seconds. (symbolized by ″)
For example, we read 35° 27′15”as “35 degrees, 27 arcminutes, 15 arcseconds.”
To help solidify our understanding of angular size a little bit. Let’s say you hold a coin in front of one eye, it can block your entire field of view. But as you move it farther away, it appears to get smaller and it blocks less of your view. The coin’s true size obviously does not change; what changes is its angular size— or the amount of space it appears to cover in your field of view, expressed in degrees.
If you'd like to support me in this adventure of creating a home-made Masters level course for everyone, please consider donating $1/month at patreon.com/spacebutmessier
In the last lesson, we talked about the night sky, so everything you see when you look up at night, but how do we really take what we see and turn it into meaning? What information do all of those sights give us?
Today, we’ll be talking about our local sky. How we measure and map our place in the heavens.
In the last video, I told you about the celestial sphere, the useful tool we have for visualizing the sky as a map. But what do you actually see when you go outside? You don’t see a sphere. You see a dome, and depending on where you live, that “dome” has different stars visible at different times. Let’s go over some key terms:
The boundary between the earth and thes sky is called the horizon,
The point directly overhead is the zenith.
The meridian is an imaginary half circle stretching from the horizon due south, through the zenith, to the horizon due north.
Our lack of depth perception on the celestial sphere means we have no way to judge the true sizes or separations of the objects we see in the sky. However, we can describe the angular sizes or separations of objects without knowing how far away they are. The angular size of an object is the angle it appears to span in your field of view.
For example, the angular sizes of the Sun and Moon are each about 1/2°,
Note that angular size does not by itself tell us an object’s true size, because angular size also depends on distance.
Check this out, The Sun is about 400 times as large in diameter as the Moon, but it has the same angular size in our sky because it is also about 400 times as far away.
You can actually estimate angular size with your hand. Stick out one finger at arms length and that’s about 1 degree wide. And you can think of 1 degree as 1 slice of a pie with 360 slices in it. And your fist, is approximately 10 degrees. To go a little deeper into the measurement of the sky, since some stars appear so small, we need to break things down a little more.
We do this by dividing 1 degree into 60 arcminutes (symbolized by ‘) , and 1 arcminute into 60 arc seconds. (symbolized by ″)
For example, we read 35° 27′15”as “35 degrees, 27 arcminutes, 15 arcseconds.”
To help solidify our understanding of angular size a little bit. Let’s say you hold a coin in front of one eye, it can block your entire field of view. But as you move it farther away, it appears to get smaller and it blocks less of your view. The coin’s true size obviously does not change; what changes is its angular size— or the amount of space it appears to cover in your field of view, expressed in degrees.
If you'd like to support me in this adventure of creating a home-made Masters level course for everyone, please consider donating $1/month at patreon.com/spacebutmessier
