Physics 101 - Astronomy - Spring 2019

Class notes for day 2, Jan. 17, 2019

This second lecture covered the Celestial Sphere, the motion of the Earth, and the difference between Solar and Sidereal Days. 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 (this is not as detailed as the Openstax book, but it is short). 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 Earth's 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. To visualize this, 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 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.

See the Powerpoint for some discussion about use of star charts from the website heavens-above.com.

Long-term changes (many years) result from the precession of the Earth as it rotates around an axis that slowly changes direction. 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. See the Powerpoint for some figures showing this long-term motion of the Earth's axis.

I have some slides for the phases of the Moon right at the end of the PowerPoint, but see Day 03 notes for this.

There will be a total eclipse of the Moon on Sunday night, starting after 9:30 p.m. Take a look outside every half hour or so. With luck, there will be some breaks in the clouds and it will be easy to see. For information, see https://www.skyandtelescope.com/observing/your-guide-to-januarys-supermoon-total-lunar-eclipse/

In the OpenStax Astronomy book, available from https://openstax.org/  you should read these sections for the first week (day 1 and 2):

Section 1.1             pp. 11-13
Section 2.1             pp. 31-41
Sections 4.1 - 4.3   pp. 103-117
and I recommend that you read the rest of Ch. 1.