Physics 101 - Astronomy - Spring 2019

Class notes for day 1, Jan. 15, 2019

Class was interupted by TWO fire alarms today, so I did not get as far as I wanted. We will do catch-up on Thursday. Nevertheless, I will post the notes for the material that I intended to cover. The audio for today ends when you hear the fire alarm go off.

 These brief class notes are really just an outline of the lectures that I give in class. They complement the material in your textbook. I will just list the figures that I presented in class, and you will have to look at the textbook to see the figures, or look in my PowerPoints.

This first class was an introduction to the course, covering the syllabus, and the features of your text. The text below is basically from the PowerPoint that I showed. This material is in section 2.1 of the online textbook.

Humans can see about 6000 stars in the night sky (with good vision and a very dark clear night), but you would need to look at various times of the year and go to the southern hemisphere to see all of them. These have been grouped into constellations (88 in the current system). Most have old names from mythology; those in the southern hemisphere have Western names. Notice how the constellations can be used to divide up the sky into regions, with boundaries that appear like political boundaries. This is a way to divide up the sky and assign names to the stars.

Here are some reasons for inventing constellations:

Story-telling, mythology, religious rituals.
Culture and religion
Navigation, for example, the pole star, Polaris, can be used to determine the direction toward the North pole, and a star named Hydra may have been used by Minoan sailors to get East-West directions (around 2400 BC).
Polynesian sailors used celestial navigation on their long voyages to new islands.

Constellations can be used to divide up the sky into regions, which is useful for making catalogs of stars and finding them for observation and study. There are 88 constellations in the current system.

We will consider a familiar constellation to help us understand the celestial sphere and the relative motion of the Earth and stars. We looked at the constellation Orion as an example. We could be using the word "Orion" to refer to an asterism, the pattern of a few stars in the sky, or we might be using "Orion" to refer to the region on the celestial sphere, the constellation Orion.

You might also look at some figures from chapter 2 on Motion of the Stars and the Sun from the Open Course: Introduction to Astronomy, which give additional detail.

We used the constellation Orion as an example to show how stars at very different distances actually appear as points on a "celestial sphere". Fig. 2-3 shows the celestial sphere, but to understand it in more detail, see the link above, which defines the celestial poles and equator, and describes the correspondence between latitude (on the surface of the Earth) and declination (on the celestial sphere), and between longitude (on the surface of the Earth) and right ascension (on the celestial sphere). The discussion is a bit terse, but it expands on the details that you see in fig. 2-3 of the text. Or see my Powerpoint.

The celestial sphere appears to rotate around us at night, because we might think the Earth is stationary. But you know that it is the Earth that is rotating, and not the rest of the Universe.

The Southern Sky is shown in a time exposure in Fig. 2-4, which shows the apparent motion of the southern part of the celestial Sphere around the South Celestial Pole.

Declination and right ascension are used to indicate positions on the celestial sphere. They correspond to latitude and longitude on the surface of the Earth. On the celestial sphere we use declination like we use latitude on the Earth. We use right ascension like we use longitude on the Earth, but it is measured in hours, minutes, and seconds. So instead of having 360 degrees (in ordinary angle units) around the celestial equator, we have 24 hours (units of right ascension). It is somewhat analogous to the time zones on the Earth. This alternate scheme of using hours for right ascension is used to align telescopes with a star and follow it as the Earth rotates (once every 24 hours), so it was convenient to use 'hours' instead of degrees for this angular measurement.