Why do we define the forward motion of the sun on the celestial field as eastward?

Question

The sun rises in east and sets in west, so we perceive the sun as moving westward... why is it defined as eastward on the celestial field? I am having a hard time picturing the motions in my head?

And by forward motion I don't mean the sun actually moves, but how we perceive its motion. I think it's a question of perspectives?

Answer 1

Adding potentially to the confusion is the east and west on the celstial sphere are opposite what we are accustomed to relative to north and south. When we look at a map, and we orient it so that north is up, west is to the left, and east to the right. When we look at a star map, east is to the left and west is to the right. If you think about the perspective difference between looking down at a map, and up at the sky, this makes sense, but it can be confusing at times.

Answer 2

The Sun's daily motion is to the west. It rises in the east and moves to the west and sets. So do all the stars. This motion is caused by the Earth rotating once a day on its axis. But the Sun also moves very slowly against the background of stars, taking a year to complete a circuit of the sky. This is due to the Earth's yearly orbit around the Sun. As a result, the length of a day is different depending on which reference point you use. It takes 24 hours for the Sun to rise, set and rise again, but it only takes 23h 56m 4s for a star to rise, set and rise again. The extra 3m 56s the Sun requires is due to it's slight motion against the background of stars in a day. Since the Sun takes a little longer to come up each morning than the stars do, it is drifting back to the East slightly as compared to the stars.

Answer 3

For our purposes, it is perfectly OK to think of the sun as moving. The DAILY movement of the sun is westward in the sky. The YEARLY movement is eastward, and by that I mean each day the sun is a little more east than it was the day before.

If you could see the stars behind the sun (if the atmosphere didn't block your view) you would see the sun each day against a slightly different background of stars, and you would see that the sun moves eastward each day against the background. By how much? Well, the sun has to go all around (360 degrees) in one year, and a year has around 360 days, so it's about one degree of arc each day, or 30 degrees each month.

Of course, you can't see the sun against its background, but you can do almost as well. You can observe stars that are exactly "opposite" the sun, meaning if you draw a line from the sun through the earth, the line will eventually reach some star that is "opposite" the sun. That star will be high in the sky at midnight, just as the sun is high in the sky at noon (high in the southern sky for northern observers; high in the northern sky for southern observers).

Each night, the line from the sun through the earth will reach a different point in the sky, one degree of arc east of where it was the night before. In one month, the point will have moved 30 degrees east of where it was the month before, or 1/12 of the way around. If you go out tonight at midnight and take note of some star high in the southern (northern) sky, then go out one month from now at 10PM, you will see that same star in that same spot.

Answer 4

I have had this same question in pondering because of my internal compass being a bit off kilter at times.

Answer 5

the sun doesn't move, the earth does...thats why...