I was just outside with the dog, and I was looking at the stars in the sky. I can tell they have moved because Orion’s belt is almost on the other side of my house then they were in the the summer, so the other stars in the constellation aren’t drowned out by the light from the city (Atlanta) close to me.
It got me to think, how much of the stars movement in the sky during the year is from the movement of the Earth around its orbit, and how much is because of the tilt of the Earth on its axis? I would think most of the movement is because of the tilt, because I would think that the orbit around our sun is minuscule compared to the distances of the stars in the sky.
So, I guess my question is, if the earth had no tilt on its axis, would the stars move noticeably during the year?
It’s from the orbit. The distance the Earth moves in its orbit each year is vastly larger than the tilt of its axis.
Think of it like this. Imagine yourself in a circular art gallery. There are 6 paintings on the wall here, by daVinci, Titian, Dali, Van Gogh, Gauguin, and Picasso (I didn’t say it was a logically arranged art gallery). Now, begin at daVinci and move clockwise around the gallery. You’ll come to Titian next, then Dali, then Van Gogh, then Gauguin, then Picasso, and then you’re back to daVinci. Same way with the stars. In the winter, our nightside faces Orion. In the summer, it faces Scorpio. This is because Orion and Scorpio are on opposite sides of the sky (I don’t say “the galaxy” because both of them (and most of the constellations we see) are in the same general area of the galaxy, off to the side in the hinterlands that our solar system occupies. The center of the galaxy is more or less in the direction of Sagittarius, which is a summer constellation (right next to Scorpio), so in the winter, our nightside is facing more or less out the back of the galaxy).
I’m also leaving out the fact that Orion ceases to be Orion (and Scorpio ceases to be Scorpio) once you get far enough outside the Earth-centered perspective. Most of what we call constellations are actually only associated visually. The actual stars are often many, many lightyears away from each other in the dimension of depth.
The Earth rotates in 24 hours, revolves around the sun in 365 days, and tilts on it’s axis in 25000 years. Logically, which would you think has the most effect on your perception of the night sky?
If the Earth had no tilt to its axis, then the movement of the stars would be completely attributable to the movement of the Earth around the Sun (you knew that already). That would mean that nothing horizontal would change, only your vertical view would change.
Imagine a bus trip around a special monument (your choice). You see the monument through the left window of the bus. You go round and round. The features of the surrounds through the right side window zip by with each trip. You might notice the photo shop, the dry cleaner, the grocery store etc (in that order) for each trip.
Now imagine that your bus is on a swivel so that it tilts on a pivot with an angle of 23 degrees in exact sync with its rotation around the monument. You will notice the same shops going by in the same order BUT you will have to look UP or DOWN from the window of the bus to see the stores. They have not moved but you have moved because you are constantly swivelling up or down like an amusement park ride.
The end result is that if the Earth had no tilt, the stars would travel in exactly the same path, and they would not stray off the zenith (point above your head) at anytime during the year).
The stars apparently run circles around our rotational axis, and this is the only movement you can see with the naked eye or even good telescopes.
Our rotational axis is only tilted from the perspective of where the sun is. It’s easiest to just think about the axis as where it is, and the stars circling that because of our spin. Define coordinates using our North and forget the tilt.
Another issue is that at different seasons, daytime and nighttime correspond to different angular positions. But the stars are still there, and you can still see some of them with a telescope, especially if you use color filters to hide the blue sky, or if you look from a balloon or jet plane at high altitude.
No, I don’t think he was. He mentioned stars at different times of the night seen at differnt points in the sky (or at least I thought he did, based on his OP).
If I was wrong, then fine.
But (the way I read it) was:
I walk the dog at 6pm in November.
I walk the dog at 6pm in January.
This not correct because there will always be apparent, diurnal motion as long as the earth turns upon its axis. The diurnal motion, however, will be parallel to the horizon. IMAX did a film at the south pole where they demonstrated this. Using one of those decorative, reflective balls for gardens as a spherical, convex mirror…the apparent motion of the sun was readily filmed (over time) as it tracked across the sky.
At the pole, the tilt of the earth has no effect, but the sun (moon, planets, and stars) STILL exhibit an apparent motion parallel to the horizon. The sun and other heavenly bodies make (virtual) circles across the sky instead of exhibiting a perceptive change in elevation relative to the horizon.
Not entirely true. Good telescopes can detect the change in star’s position due to the earth’s movement in its orbit. That change is called parallax, and its measurement has helped us determine some distances to the stars. But, it has a limited range for valid results. Other techniques had to be found when the angle of parallax becomes too small. - Jinx
Yes, it is a little known fact that this, indeed, DOES affect a star’s apparent position. It is called the “aberration of light”, and its affect is very small. Here’s a quote from Wiki:
“The aberration of light (also referred to as astronomical aberration or stellar aberration) is an astronomical phenomenon which produces an apparent motion of celestial objects. It was discovered and later explained by the third Astronomer Royal, James Bradley, in 1725, who attributed it to the finite speed of light and the motion of Earth in its orbit around the Sun…”
I think we’re all making this way too complicated.
The OP isn’t an astronomy hobbyist.
OP: You look at the stars at night. What is “night”? It’s when your spot on Earth is facing away from the sun. Between today and 6 months from today (past or future), the Earth will be halfway around its orbit. So the direction that is [away from the sun] will be 180 degrees different. THAT’s the vast majority of what you’re seeing.
As a made up example, let’s say that on Jan 1 at midnight in Atlanta the constellation Bogus is directly overhead. 3 months later on Apr 1 that same constellation will be just rising in the east at midnight. 3 months later on Jul 1 that same constellation will be directly beneath your feet. And 3 months later on Oct 1 that same constellation will be just setting in the west at midnight.
You’re right that the orbital distance we move from winter to summer is a tiny distance compared to the distance to the stars. That affects the apparent position of the stars by a tiny amount; far smaller than you can notice even though instruments can measure it.
The important thing is that you’re facing a different direction each night at the same time as we move around the orbit.
I think some people (and I’m not sure whether or not that includes the OP) mistakingly believe that the axle tilt is an actual yearly movement back and forth like nodding your head.
Your right, I was talking about the axis tilt that causes the seasons.
Thanks, that explains it to me perfectly. What made me think of the questions was that Orion was in a different side of my house when I was outside last night. Normally most of the Orion constellation is washed out by the light pollution from the City of Atlanta so all I can really see is the belt. Last night it was very clear, and Orion was away from the lights of Atlanta so I was actually able to see the constellation (I still don’t see the whole man, but then I can’t see the man in the moon either).
When I was asking the question, I honestly didn’t think of the fact that we are facing a totally different direction at night when we are on opposite sides of our orbit. I know it intellectually but wasn’t even thinking about it.