Astronomy C

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SuperAJ
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Re: Astronomy C

Post by SuperAJ » January 13th, 2014, 8:37 pm

I hate the long tests that do not pose difficult questions, but are big in terms of volume. This sometimes hurts the person that writes out long and thought-out responses to questions.

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Re: Astronomy C

Post by nomynameisnotkevin » January 17th, 2014, 6:05 pm

AlphaTauri wrote:I had the key mostly done at some point, but I uh, don't know where it went... I'll try to redo it within the next month or so (after this invitational I've got coming up).
That would be much appreciated, as there are some fantastic questions on there that I really wished I knew :P

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Re: Astronomy C

Post by Schrodingerscat » January 22nd, 2014, 7:49 am

Has anyone found a good all-in-one yet highly detailed HR diagram? The only ones I have ever been able to find might show a particular part very well, but completely neglect other parts.

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Re: Astronomy C

Post by syo_astro » January 22nd, 2014, 10:49 am

Schrodingerscat wrote:Has anyone found a good all-in-one yet highly detailed HR diagram? The only ones I have ever been able to find might show a particular part very well, but completely neglect other parts.
Personally I haven't really found any "all-in-one" diagrams. I'd recommend just having it all in your binder/laptop just in case. So that can be focused on variable stars, stellar evolution, color-magnitude diagrams for clusters, varying units for axes, focused on different mass ranges/sections, and whatever other labels you could think up or find. Hope that helps!
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Re: Astronomy C

Post by hexagonaria » January 31st, 2014, 5:44 am

Hi people.
So, I understand how astronomical coordinates are written as right ascension and declination, but I'm confused about how they can actually be used to locate DSOs. I mean, the Earth rotates on its axis every 24 hours, and orbits around the sun approximately once a year. So, wouldn't the right ascension of an object in space be constantly changing throughout the day and the year? So if I told you to find an object at RA: 02h 19m 20.70s and DEC: -02° 58' 39.51" , when would you take that measurement?
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Re: Astronomy C

Post by Schrodingerscat » January 31st, 2014, 5:53 am

hexagonaria wrote:Hi people.
So, I understand how astronomical coordinates are written as right ascension and declination, but I'm confused about how they can actually be used to locate DSOs. I mean, the Earth rotates on its axis every 24 hours, and orbits around the sun approximately once a year. So, wouldn't the right ascension of an object in space be constantly changing throughout the day and the year? So if I told you to find an object at RA: 02h 19m 20.70s and DEC: -02° 58' 39.51" , when would you take that measurement?
There are two different coordinate system: Horizontal and Equatorial.
Horizontal coordinates use azimuth and altitude and are relative to a observer's position and time.
However, equatorial coordinates, with RA and azimuth, are more or less absolute. The celestial equator and poles are defined with a declination of 0 and +/- 90 degrees respectively, and RA is defined from the equinox, making it mostly time independent for practical purposes.

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Re: Astronomy C

Post by EastStroudsburg13 » January 31st, 2014, 6:25 am

You can take that measurement at any time. This is because right ascension moves along with the celestial sphere, so as an object moves from the east to the west, the right ascension value that it corresponds with also "moves". Much like longitude, even though the Earth rotates, the network of longitudes also rotates along with it.

Because of this, it might seem impossible to know how to figure out the right ascension at any given time, but luckily there are two values that help in this regard. These two values are hour angle and sidereal time. Hour Angle is a measure that shows an object's relative position to the observer. Like RA, it ranges from 0h to 24h, with 0h at the star's highest point, 6h to the right, 12h at the lowest, and 18h to the left. In the following diagram, the circumpolar star would have an hour angle of 0h at its highest point and 12h at its lowest point:
Image

Local sidereal time (LST) is a way of measuring time with regard to the stars. One full sidereal day is the time it takes for the Earth to completely rotate back to the same position of the stars. On Earth we typically use solar time to measure time, i.e. how long it takes for the Earth to rotate back to the same position relative to the Sun. However, since the Earth also revolves around the Sun, it takes a little extra rotation to get back to the same position. Because of this, a sidereal day is shorter than a solar day. This diagram may be helpful, where 1 -> 2 is a sidereal day, and 1 -> 3 is a solar day:
Image

Now how are these all related? Well luckily there is a simple mathematical relationship:

This really illustrates how the stars move through the sky. First, note that at any given sidereal time, the night sky will always look the same. Even though the solar time might be drastically different, the stars' positions at the same LST will always have the same HA, since the stars must always have the same RA. At different times of any given night, the HA changes, but since the LST changes as well, the RA remains the same for any given star. Also, at different times of the year, the sidereal time at midnight will be different, but as you might know, the sky appears different at different times of the year, so when you account for hour angles, the RA is the same!

This can be a little difficult to really grasp, since there are so many times going around, so here's an example. Beta Cassiopeiae has a right ascension of about 0. However, let's say you're stargazing and you see it appears to the direct right of Polaris. This means that Beta Cas has a hour angle of 6, and so through the mathematical relationship, you can calculate the LST to be about 18:00 (since we don't want negative time). It doesn't matter what the solar time is, it could be 6 AM or 6 PM, but whenever the hour angle of Beta Cas is 6, the LST will always be 18:00. This makes Beta Cas helpful for finding sidereal time, since it is easy to find (it's the right-most star in the W/left-most star in the M) and has a RA of about 0.

This is probably way more than you wanted, or needed, to know. But basically, if you were to take that measurement, you can find the sidereal time, and then using the formula, calculate the hour angle, which is where in the sky you would expect it to be. This allows RA to be a standard coordinate for the entire world to use, without worrying about when the measurements are being taken.

Hope this helps!
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Re: Astronomy C

Post by hexagonaria » January 31st, 2014, 2:59 pm

EASTstroudsburg13 wrote:You can take that measurement at any time. This is because right ascension moves along with the celestial sphere, so as an object moves from the east to the west, the right ascension value that it corresponds with also "moves". Much like longitude, even though the Earth rotates, the network of longitudes also rotates along with it....
You are amazing, East. I had some big misconceptions here, but the way you explained it made a lot of sense. I don't really know if this sort of knowledge will be terribly pertinent for the competition, but not understanding it was bugging the hell out of me, so thank you!
If you ever want to know anything about Rocks and Minerals, shoot me a message ;)
And thank you as well SchrodingersCat
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Re: Astronomy C

Post by EastStroudsburg13 » January 31st, 2014, 3:23 pm

No problem, really! We covered a lot of that stuff in my Observational Astrophysics class last semester so it's pretty fresh in my mind. It also makes a fairly good party trick to be able to tell the sidereal time at night, even though people are rather unlikely to care about sidereal time and are more interested in constellations. :roll:
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Re: Astronomy C

Post by identicalgamer » February 5th, 2014, 8:15 pm

Hey, I am kind of new to this event and was wondering if anyone would be able to look over the information I have gotten together so far. In my state this is a binder event (no laptops) meaning that all of the information that I have so far is going to have to be printed off and put inside of a binder.

Here is my work so far:
https://docs.google.com/document/d/1bWY ... sp=sharing

Please leave comments on it or tell me what you think here.

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