Physics 101 - Astronomy - Spring 2009
Class notes for day 1, Jan. 20, 2009
These brief class notes are really just an outline of the lectures that I give in class. They complement the material in your textbook. Since I am not allowed to post the figures from the textbook on the web (they are copyrighted), I will just list the figures that I presented in class, and you will have to look at the textbook to see the figures.
Day 1 notes for Jan. 20, 2009.
This first class was an introduction to the course, covering the syllabus, and the first half of the Introduction in your text.
The grand scale of the universe is illustrated in the fig. on p. 1, which motivates the need for scientific notation (we'll cover this later).
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, ritual:
Culture and religion
Navigation:
The pole star, Polaris, can be used to determine the direction toward the
North pole.
Hydra may have been used by Minoan sailors to get East-West directions (2400
BC).
Polynesian sailors used celestial navigation.
Constellations can be used to divide up the sky into regions.
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.
At this point in the lecture I showed some figures from chapter 2 on Motion of the Stars and the Sun from the Open Course: Introduction to Astronomy, which is available on the web and which you might want to read in parallel with section E1 of your textbook.
We used the constellation Orion as an example to show how stars at very
different distances actually appear as points on a "Celestial Sphere". See
figure E.3 in your text. Fig. E.4 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 and
declination, and between longitude and right ascension. The discussion is
a bit terse, but it expands on the details that you see in fig. E.6 of the text.
The Celestial Sphere appears to rotate around us at night. But you know that it
is the Earth that is rotating, and not the rest of the universe.
To observers who think the earth is stationary, the celestial sphere appears to be rotating.
The Northern Sky, in a time exposure in Fig. E.5, shows the apparent motion of the northern
part of the celestial sphere around the Pole star, Polaris.
Right Ascension and Declination 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 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.
The next section, E.2, shows how the Earth's orbital motion around the Sun affects our view of the night sky. We can understand this better by considering that the Earth rotates around its own axis, and also revolves around the Sun. Remembering that the Sun goes down in the West at sunset, we can see that the Earth rotates to the East, or counterclockwise (CCW) as viewed from above the North pole. To motivate this idea of CCW rotation of the Earth, I found the latitude and longitude of Macomb on the web site www.lat-long.com Then I put these values into the "Earth-Moon viewer" at http://www.fourmilab.ch/earthview/vplanet.html Start the view at an elevation of 100,000 km above Macomb, IL at midnight UTC, June 7, 2005 (in local time, 7:00 p.m., June 6, 2005). I produced a series of images of the Earth rotating, with the U.S. going into the night side of the planet, one hour at a time. This helps us remember which way the Earth rotates.
See Fig. E.7 of the text, which is a sketch of how the Earth rotates AND revolves around the sun at the same time, both in CCW directions as viewed from above the North pole. Imagine yourself at point A, with the sun directly overhead. It's noon, and after one SOLAR day, you are again at point A with the sun overhead and 24 hours have passed. However, the Earth had to rotate more that 360 degrees. Due to orbital motion, Earth had to rotate 360.986 degrees, because it has gone along its orbit, and it had to rotate the additional angle shown in Fig. E.7, which is 360/365 degrees, or 0.986 degrees. (Earth revolves 360 degrees in 365 degrees, hence about 1 degree per day.)
The sidereal day is, by definition, the time it takes for the earth to rotate around and come back into alignment with the stars. This is a rotation of exactly 360 degrees and this takes 3.9 minutes less than 24 hours. The conversion factor is: 1 sidereal day = 0.9973 solar days (see Table 2B on p. A-4 in the Appendix at the back of the textbook). So what makes this distinction important? The difference between the solar day and the sidereal day means that the Sun and the stars appear to be going around the Earth at different rates. The Sun goes around in 24 hours. Stars go around in 23 hours, 56.1 min. So the Sun is not in the same place on the celestial sphere day after day.
The Zodiac in shown in Fig. E.8, the ecliptic in Fig. E.9, and the origin of the seasons is shown in Fig. E.10. My class discussion followed the text, but then I also used some figures from the Motion of the Earth section of the Open Course: Introduction to Astronomy, which might be useful to review. You can also go back to sections 2.3 through 2.7 of the prior chapter on Motion of Stars and Sun of the Open Course: Introduction to Astronomy to amplify some of the ideas about the ecliptic.
I pointed out that the picture of a celestial sphere and the ecliptic allow us to describe short-term (daily) and medium term (yearly) motion of stars in the sky. Seen from far above the North pole, in the scheme having a fixed celestial sphere, the Earth appears to be rotating counterclockwise (abbreviated CCW). On the other hand, for an observer standing on the earth’s North pole, the entire celestial sphere appears to be rotating counterclockwise (CCW) as well, as seen in Fig. E.5 on p. 6 of your text. To visualize this, look again at Fig. E.4 and put yourself either at the Earth's North pole, or out in space at the north Celestial pole. Now to understand the yearly motion of the sun on the celestial sphere, look at Fig. E.8 and also this figure from Richard Pogge's Solar System Astronomy course at Ohio State. These figures show that the sun appears to move through the constellations of the Zodiac, about one per month, as it travels around the ecliptic.
Long-term changes (many years) result from the precession of the Earth as it rotates around an axis that slowly changes direction. This is shown in Fig. E.12 on p. 11 of your textbook. The axis of the Earth precesses with a period of 26000 years and an observer on the North pole would see the zenith move around a circle in the sky over this long period of time. In our lifetime this will not be a significant issue, however.
Day 2 (Thursday, Jan. 22), we will cover the Moon's rotational and orbital motion and also parallax. We will try to finish the Introduction. So in this order,
Get the textbook and read the Introduction.
Review these notes.
Look at the material in this on-line description, but just skim it, since we won't need to know every detail: Motion of the Stars and the Sun and the next section Motion of the Earth.
