ASTR 100 Section 2 Fall 2003

Responses to Student Questions

Often we are asked questions that are of general interest, so I will post the questions and their answers here.

Oct 30th Hot, low density gas and hot, high density gas comparison.
Q: How would the emission of a hot, low density and a hot, high density gas compare?

If a gas is low density (rarefied) and hot, it can emit spectral radiation. If a gas is hot and has a high density, the large number of atoms in the gas, cause the emitted photons to get repeatedly scatter, and that creates a blackbody emission spectra. See Section 4.4 for more details.

Sept 4 How do I get to the homework?
Q: I was confused about how to retrieve the current homework. Could you please explain how to find it on the web?

Go to Click Login. Click Login again. Enter your NetID, then associated password.

That should take you to a webpage with all of your courses that have WebCt pages. Click on ASTR 100 Section 2. Click on Quiz. Click on HomeWork#1. Click on your choice of answer and "Save answer" for each question. When you have double checked your answers, and you want to submit your homework, then click at the end of the page, "Finish".

If this does not work for everyone, let me know as soon as possible.

Sept 6 What's the difference between Sidereal and Synodic months for the Moon.
Q: What is the difference between a sidereal month and a lunar month. Doesn't the moon change phases because it orbits the earth?

Yes, the phases are a result of the Moon's orbit around the Earth (or how much of the sunlit part of the Moon we see), BUT it is also depends on the Earth's orbit around the Sun.

Refer to Figure 1-21 in your book. A new Moon is the phase we see when the Moon and Sun are in the sky together. The Sidereal month is the time it takes for the Moon to orbit the Earth once-- 27.3 days. If the Earth did not orbit the Sun, this would also be equal to its Synodic month. However, as the Earth DOES orbit the Sun, the Moon has to travel a little further to reach the same Phase (again see Fig 1-21 in your book). Remember the phase depends on the relative position of the Earth, Sun, and Moon. So each month the Moon has to catch up, and the Synodic period is slightly longer than the Sidereal Month-- 29.5 days.

Sept 8 What is the Ecliptic anyway?
Q: I don't really understand what the ecliptic is; is it the same as the celestial equator?

No. The celestial equator is the imaginary line on the sky if the Earth's equator were projected onto the celestial sphere of stars. If you visited the Equator, say Ecuador, the celestial equator would always be at your zenith.

Now the ecliptic is the path where the Sun appears to move during the Year, projected onto the celestial sphere. Figure 1-15 in the book, shows this best. The Earth is shown with the celestial equator marked in a bluish-gray color. The ecliptic is shown as a tilted yellowish-green circle. The ecliptic is tilted with respect to the celestial equator by 23.5 degrees which is the tilt of the Earth's orbit that represents itself by this effect. If you visited Ecuador, the Sun (and thus the ecliptic) would only be directly overhead twice a year, at the Autumnal Equinox and the Vernal Equinox.

So as shown in Figure 1-15, the Sun is above the celestial equator during the Summer solstice by 23.5 degrees and the Sun is below the celestial equator during the Winter solstice by 23.5 degrees.

If the Earth's orbit were not tilted, the ecliptic and the celestial equator would be the same thing.

Now in class today, I discussed the ecliptic plane. The ecliptic plane is the just the plane which holds the ecliptic, see Figure 1-11b in the book.

Leslie Looney
Last modified: Mon Sep 8 15:12:16 CDT 2003