The Motion Of Stars, Planets, The Sun And The Moon Over The Sky

It is well known that the stars rise in the east and set in the west and that the southern hemisphere sky is different from the one in the northern hemisphere. But why is that so?

Why We See The Stars We See In The Night Sky

The Earth and all the planets in our solar system orbit the sun in almost the same plane because they were all formed from the same cloud of gas and dust(because of centrifugal forces the spinning cloud flattened out into a plane). (Earth is defined to have 0 degrees orbital inclination(that's what we define to be the ecliptic) and all other solar system objects orbital inclination is measured relative to Earth's. The planets only slightly deviate from the ecliptic, Mars for example has 1.85 degrees inclination.) This is why the sun and the planets all take the same path over the sky. But The Earth's axis of rotation is not at 90 degrees to this plane. The Earth is tilted by about 23.5 degrees. This is thought to be because of collisions with objects like asteroids in the early days of the solar system. This tilt causes the seasons. But it also causes earthlings to see a slightly different sky then martians because Mars is also tilted but at a different angle.
But the solar system is also tilted relative to the plane of rotation of our galaxy the milky way by 60.2 degrees. This causes the southern hemisphere to see the center of our galaxy while the northern hemisphere looks towards the outer edge of the galaxy.(The solar system is tilted because it did not form at the same time as the Milky Way. The cloud of gas and dust that would become the solar system randomly had this tilt)
But one day the northern hemisphere will see what the southern hemisphere sees and vise versa because its like the seasons where part of the year the south points toward the sun so it's summer there and the other part of the year the north points toward the sun and its summer there because the Earth's tilt does not change while orbiting the sun. For the solar system it's the same. For a few million years one side of the plane looks at the center of the galaxy and then for another couple of million years the other side of the plane points in that direction.

The Earth's tilt also causes the stars to change with the seasons. As can be seen on the image above, when the earth is on the left of the image the nightside points towards the left while when the Earth is on the right the night side points towards the right. So at different seasons the earth's night side points in totally different directions.

Why Stars Appear To Move Over The Sky

The stars, planets, the sun and even the moon all rise in the east and set in the west. This is because the Earth rotates from west to east or if you could look down on the north pole it would look like it was turning counterclockwise. Since we think that we are stationary it looks like the stars are moving towards the west, but that's only because we are actually rotating towards the east.
At the poles the stars don't rise or set because the axis of rotation of the Earth goes through the poles. And at the equator all stars set because the equator is parallel to the axis of rotation.
The path the stars take over the sky is curved because the earth is round.

Rising And Setting Point At Same Location

All stars will rise in one spot in the east and set at that same spot but in the west. So for example, if a star rises in the south east it will set in the south west. This is because the angle at which you look at the sky does not change.
If you stay at the same latitude the position where stars set(or rise) does not change. 2 people at same latitude, but different cities will see the same night sky because of earth's tilt. The 2 people look at the sky from the same angle.

The time at which they rise and set changes because the path the sun takes over the sky changes. The setting(and rising) position of the sun changes by ~1 degree every day because the Earth is tilted relative to the plane of the solar system. In winter the hemisphere which has winter has a smaller region where there is sunlight.

Rising And Setting Point At different Locations

If you change latitude the sky changes because the angle at which you look at the sky changes. If you start at the north pole and you go towards the equator less and less constellations from the north are visible and more and more stars that are visible at the equator appear. The same goes for starting at the south pole and going to the equator.

If you start at the south and go towards the equator all the stars that are visible at the south pole start setting(and rising) more and more south. If you start at the north and go towards the equator all the stars that are visible at the north pole start setting(and rising) more and more north. For the stars which are overhead at the equator and low or not visible at the poles they also set(and rise) more and more in the direction of the equator as you move away from the equator. So if you start at the equator and go north the stars visible at the equator set(and rise) more and more south. Same for if you go south(equator stars set more and more north).

The Moons Path Over The Sky

The moon has a path over the sky which is always very similar to the sun. So as the sun's path changes over the year the moon's path changes in the same way. This is because the moon's orbital plane is similarly aligned with the solar systems plane(defined to be Earth's orbital plane)(ecliptic). In fact the angle between the moon's orbital plane and the plane of the solar system is 5.1 degrees. This is not much so the two paths are very similar but it still makes a difference. (This tilt is thought to be caused by close encounters with asteroids which through their gravity influenced the moons orbit.) First the moon's path is close to one side of the sun's path, then it crosses that path and is close to the other side and then it crosses the sun's path again and is close to the first side again. The 2 paths cross because part of its orbit the moon is 5.1 degrees above the plane of the solar system and the other part its 5.1 degrees below.
This 5.1 degree tilt is also why we do not have an eclipse(solar or lunar) happening every full moon and every new moon. Eclipses only happen at new and full moon because then the moon is either opposite the sun or on the same side as the sun. If the moon's orbit was perfectly aligned with the solar system's plane, we would have two eclipses(one solar and one lunar) every month. But this is not the case. We only have an eclipse if the moon crosses the plane of the solar system and it is full or new.
The Moon moves about 13 degrees eastward every day(some times a bit more sometimes a bit less because the moon's orbit like earth's orbit is not a perfect circle)(360 degrees/13 degrees=~27 and a moon orbit is slightly more than 27 days).(Since the earth turns around its own axes we see the moon rise and set each day even tough it does not move that much each day.) These 13 degrees eastward is why starting at new moon it always rises about 52 min later everyday until the next new moon.(The Earth moves about 4 degrees every minute so it has to turn 13 degrees more so that we see the moon at the same place(13*4=52). In a month the sun's path does not change very much, but because the moon moves 13 degrees east every day it rises at the same time as the sun when it is new moon (which is why we can't see it) and then rises when the sun sets at full moon(it is full because it is opposite the sun).

The moon's orbital period does not fit nicely a certain number of times into Earth's orbital period around the sun. So every year the moon does 12 orbits and a partial orbit(technically the moon does 13 and a bit orbits if measured relative to the background stars, but since Earth goes around the sun the angle between the sun and the moon changes so the moon has to travel further to have the same phase. There are 12 and a bit phase cycles in a year). These partial orbits add up so that after 18 years and 11 and 1/3 days the Earth's year and the moon's orbital period align again. This is why every 18 years and 11 and 1/3 days the moon will have the same phase as it had on the same day 18 years and 11 and 1/3 days ago. This is called the Saros cycle.

The Planets Path Over The Sky

The planets follow a similar path over the sky as the sun because they all orbit on the same plane as Earth around the sun. That's because they all formed from the same cloud of dust and gas which was flattened by centrifugal forces and was spinning anticlockwise so all planets and the sun rotate anticlockwise around their axes, and they orbit the sun anticlockwise.(Venus rotates clockwise because it's thought to have been tipped over by an asteroid impact.)

Venus and Mercury(the inner planets)

Venus and Mercury orbit the sun faster than Earth does which is why they sometimes look like they are going backwards a lot compared to the rest of the sky. Since they are close to the sun from our point of view they are always visible either around sun set or sun rise but never in the middle of the night.
After passing behind the sun they are moving westward. As they catch up with Earth in their orbit they appear to go eastward and are visible in the evening(in the west). Then as they pass in front of the sun(so between sun and Earth) they are not visible or create a transit. Then they reappear in the east in the morning sky as they overtake Earth in there orbit and they seem to go westward. Then ones they are far "ahead" the angle has changed because orbits are circular so they seem to go east before disappearing behind the sun again.

The outer planets

They orbit the sun slower than we do. So they seem to move in the opposite way compared to the inner planets. When they come out from behind the sun after being on the opposite side of the sun relative to Earth they are in the morning sky. Then they slowly start appearing earlier and earlier in the night as Earth cathes up with them(apparent eastward motion). Then as Earth overtakes them they appear to move westward or "backwards" until the Earth is more then a quarter orbit away from them. At that moment it goes back to eastward motion because since the orbit is circular the angle has changed so Earth is back to "catching up".
When they are on the same side of the sun as Earth they are visible in the evening and when they are on the other side of the sun compared to Earth they are visible in the morning.

planet phases

If you where standing on any planet in the solar system you would see phases on all planets orbiting closer to the sun but not on the planets orbiting farther. This is because from your angle all the planets further out are always showing you the side that is fully illuminated because since you are closer in you don't see the night side from your point of view. While for the inner planets your angle is such that sometimes you can see the night side.
So from here on Earth we can see Venus and Mercury go through different phases like the moon. When they are on the same side of the sun as Earth they are in the new phase and when they are on the other side of the sun compared to Earth they are in the full phase. Like the moon their orbital plane is very close but not perfectly the same as the solar system plane(ex: Venus has a 3.24 degrees orbital tilt which is thought to be caused by asteroids which passed close by and with there gravity changed the orbit slightly.) So sometimes we can see Venus or Mercury path in front of the sun(transits) but not all the time and sometimes we can see them full while other times they are hidden behind the sun. This is for the same reason as for the moon not making an eclipse all the time. The planet has to be full or new when it is at that point in its orbit where its orbital plane and solar system plane intersect.

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Copyright © 2021 Jessica Socher ()