Understanding the Celestial Sphere: Earth, Sky, Sun, Moon

1. Astronomy notes for Phys/Geog 182 • Week 5 – Earth, sky, Sun, and Moon • Week 6 – Motion in the Solar System • Week 7 – How stars work • Week 8 – The cosmos in the large • We will use a combination of simulations and worksheets • Topics align with teaching standards

2. The recommended textbook is “Astronomy”, by A. Fraknoi, D. Morrison, and S.C. WolffAvailable free online at openstax.org

3. Openstax Astronomy Week 1 readings: Read sections 1.1 - 1.8 pp. 11-29 (skip calculations) Study section 2.1 pp. 31-41 Read sections 2.2 & 2.4 pp. 42-49 & 54-61 Read all of Ch. 4 Look at key terms and chapter summaries.

4. What can we see in the visible sky? • Humans can see about 6000 stars in the night sky (with good vision and a very dark clear night). • Some of these form patterns called asterisms. • 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.

5. The Constellation Orion, as seen in the sky and as imagined.

6. Why constellations? • Why do people invent constellations in the night sky? • Before indoor lighting, many people spent their evenings under the stars, with plenty of time to talk and observe.

7. Constellations were invented for • Story-telling, mythology, ritual • Culture and religion • Navigation and timekeeping • Mapping the sky; modern astronomy uses the constellations to label areas that are delimited by boundaries.

8. Mapping the sky: Constellations can be used to divide up the sky into regions.

9. For example, Orion can be used two ways: as an asterism. as a region of the sky.

10. The Celestial Sphere • The distance to the stars is not evident to our eyes, and they appear to be at the same distance. • If we think of this distance as a radius, the stars appear to be on a sphere, with us (on the Earth) at the center.

11. The Constellation Orion is actually three dimensional, but appears to us as a group of points on the “celestial sphere” For more images, see this linkto the “Open Course Astronomy” which has simple figures to illustrate these.

12. The Celestial Sphere appears to rotate around us at night. But you know that it is the Earth that is rotating. (link)

13. To observers who think the earth is stationary, The celestial sphere appears to be rotating.

14. The Northern Sky, in a time exposure, shows the apparent motion of the northern part of the celestial sphere around the Pole star, Polaris.

15. 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.

18. The celestial sphere is oriented with respect to the earth, with poles and an equator.

19. On the celestial sphere we use Declinationlike we use Latitude on the Earth.

20. On the celestial sphere we use Right Ascensionlike we use Longitude on the Earth, but measured in hours, minutes, and seconds.

21. For an observer on the ground, directions are defined in this figure.

24. The observer on the ground can also use angles for altitude (Alt), the angle up from the horizon, and azimuth (Azm), which is a heading (N, E, S, W etc.).

25. For more detail on Right Ascension and Declination, see this.

26. Stars are (almost) fixed on the celestial sphere and the Earth rotates inside this sphere. It only looks like the celestial sphere is rotating from our perspective if we are on the Earth.

27. Question ? • To us, which way does the celestial sphere appear to be rotating? • Hint, think about the motion of the moon and the sun during the day or night. • Now, if the celestial sphere is actually not moving, and the Earth is rotating, which way does the Earth rotate?

28. Apparent rotation of the celestial sphere is due to the rotation of the Earth. The Earth is rotating around an axis that goes from pole to pole through a center. Eventually, each day, the Sun sets in the west. If we suppose the Sun is the center of the solar system, it is fixed, so: Each point on the surface of the Earth is going east all the time (except the poles).

37. Seen from far above the North Pole, the Earth appears to be rotating counterclockwise (CCW). Sun If the Sun is directly above point A, then it is local noon there, and in 24 hours it will again be noon at that location on the Earth.

38. The Earth is also in orbit around the Sun, taking 365.25 days to revolve once around. This orbital motion is also CCW if viewed from above the north pole.

39. In 24 hours, which is called the solar day, the Earth must rotate more than 360 degrees!

40. Solar vs. Sidereal Day • The solar day is 24 hours long, by definition, but Earth actually rotates through an angle of 360.986o in order to be aligned with the Sun. This is due to the orbital motion of the Earth, which means that the Earth has to rotate an additional 360o/365 or 0.986o per solar day.

41. Solar vs. Sidereal Day • The sidereal day is, by definition, the times it takes the Earth to rotate around and come back into alignment with the stars. This is a rotation of exactly 360o and this takes 3.9 minutes less than 24 hours. • 1 sidereal day = 0.9973 solar days.

42. Why is this relevant ? • 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.

43. The Zodiac is the set of constellations that the Sun appears to go through during the course of one year.

44. On Jan. 18, 2017, about 2:30 p.m., the Sun was in Sagittarius, but we can’t see the stars in daylight. • This can be seen on a sky chart if we set the time to sometime in the day, say 2:30 p.m. using • http://www.heavens-above.com/skychart.aspx?lat=40.459&lng=-90.672&loc=Macomb&alt=215&tz=CST • Note: you may need to set the date and time to Year 2018 Month 1 Day 18 Hour 14 Minute 30 and click Update. You can also print black on white to save printer ink if you want to take a printed chart outdoors, having set the time to the evening hour when you plan to go out. • Compare with the zodiac chart in the previous slide. • Around midnight, we can go outdoors and see Cancer high in the sky, opposite the Sun from Earth. (next slide) • You can try different months to see that the celestial sphere is in different orientations during the year.

45. At night on Jan. 18 at 11:55 p.m. the sky should look like this • http://www.heavens-above.com/skychart.aspx?lat=40.459&lng=-90.672&loc=Macomb&alt=215&tz=CST • Set the time for 23:55 and notice that we see Cancer and Gemini high in the sky. • The sky chart also show us that these are relatively high in the sky at this time, near the zenith. • Depending on the date, we will see some planets along the ecliptic. • For a quick summary of visible planets, etc., see the article “This Week’s Sky at a Glance” in Sky and Telescope magazine: • http://www.skyandtelescope.com/sky-at-a-glance/

46. The Ecliptic is the path of the Sun on the celestial sphere, which is tilted with respect to the celestial equator, due to the tilt of the Earth’s axis with respect to our orbit.

47. The axis of the Earth is not perpendicular to the plane of the orbit of the Earth around the Sun. The Earth is tilted by 23.5o.

48. Seasons are due to the changing orientation of the Earth and Sun, not because the Earth is orbiting closer or further from the Sun.