Astronomy Questions

1) Can you explain the difference between sidereal time and apparent solar time?
2) When we say that electrons "excite" atoms of oxygen and nitrogen in the atmosphere, what do we mean? Does it mean that they are ionised?

A mean sidereal day is about 23 hours, 56 minutes, 4.0916 seconds (23.9344699 hours or 0.99726958 mean solar days), the time it takes the Earth to make one rotation relative to the vernal equinox.

Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day.

Auroras result from emissions of photons in the Earth's upper atmosphere, above 80 km (50 mi), from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to ground state. They are ionized or excited by the collision of solar wind and magnetospheric particles being funneled down and accelerated along the Earth's magnetic field lines; excitation energy is lost by the emission of a photon, or by collision with another atom or molecule:

oxygen emissions
green or brownish-red, depending on the amount of energy absorbed.
nitrogen emissions
blue or red; blue if the atom regains an electron after it has been ionized, red if returning to ground state from an excited state.

Answer:


http://en.wikipedia.org

I'm taking my GCSE exam next week, so I though I'd make this thread where I can ask all of my questions, no matter how stupid they are, because I'm a bit confused at the moment.

1) Can you explain the difference between sidereal time and apparent solar time?
2) [This is in the context of the aurorae] When we say that electrons "excite" atoms of oxygen and nitrogen in the atmosphere, what do we mean? Does it mean that they are ionised?

1)
Apparent solar time (or just solar time) is based on the apparent movement of the Sun around the sky. You could call it "sundial" time. One solar day is the time it takes the Sun to apparently go once around the Earth.
Sidereal time is based on the apparent movement of the stars around the Earth. One sidereal day is the time time it takes a star to apparently move once around the Earth.
The reason they are not the same is that the Earth is also moving around the Sun.
The Earth goes around the Sun once in a year which means it makes a 360 deg orbit in 365 day. That's about 1 degree a day.

So for the Earth to rotate once with respect to the Sun, it has to do its own 360 degrees, plus one more degree because it has moved 1 degree around the Sun. So the sidereal day is a little longer than the solar day.

There is an animation here to show this.

http://bcs.whfreeman.com/universe7e/content/ch02/0203003.html

My specification has a point that says "explain why a solar day is longer than a sidereal day"???

Is that because a sidereal day is one rotation relative to the fixed stars, whereas in the solar day, you have to account for the extra 1 degree?

Yes. I just read my last post and it should have said solar longer than sidereal of course. The explanation is correct just a typo on the last bit.
Yes. The earth has to rotate 361 degs to get the Sun back to the same point because the Sun has moved on about 1 deg in that time. Hence the solar day longer than the sidereal.

I've got another question:

We say that a transit is when a smaller body moves in front of a larger body, however is this about angular size or actual size? I mean would you consider a solar eclipse a transit, since the moon is smaller than the sun, but has the about the same angular size?

Edit: Oops, misread Wikipedia before; it say that it's an occulation. However what is the boundary between a transit and occultation, and I'm guessing that since it's relative to an observers position it does depend on angular size?

Probably should start numbering these questions:

4) How do you definite a superior conjunction?

My definition would be that the other body is on the opposite side of the sun and the angle of elongation is 0 degrees?

Yes. This is a standard definition.

http://www.heavens-above.com/glossary.aspx?term=superior+conjunction&lat=0&lng=0&loc=Unspecified&alt=0&tz=UCT

5)Do you know what my textbook means when it says "rarefied spherical coma"? [in the context of when a comet approaches the sun].

The wiki page explains this ok. There is even a picture of the coma round the head of the comet.
http://en.wikipedia.org/wiki/Coma_(cometary)