Motion in the Sky

1. Motion in the Sky

2. Discussion What is a year?

3. Discussion What is a day?

4. Discussion Why are there 7 days in a week?

5. Time Day – Time it takes the Sun to return to the same position above the horizon. Month – Time it takes the Moon to go through its phases. Year – Time it takes the Sun to return it the same position relative to the stars. Note: Ancient definitions differ from the modern usage!

6. Discussion Do you know how the hour was originally defined? An hour is the time it takes the Moon to move its diameter against the stars.

7. Fixed stars and Planets Although the stars change their positions over the course of a night and from night to night, their relative positions (their positions relative to each other) remain constant year after year. Planet means “wanderer.” They change their positions relative to that of the fixed stars

8. The Planets of antiquity Sun Moon Mercury Venus Mars Jupiter Saturn

9. The days of the Week Sunday – the Sun’s day Monday – The Moon’s day Tuesday – Tiw’s day – Mars Wednesday – Woden’s day – Mercury Thursday – Thor’s day – Jupiter Friday – Freia’s day – Venus Saturday – Saturn’s day

10. Ordering of the day names Planets ordered by apparent length of period of revolution: Saturn Jupiter Mars Sun Venus Mercury Moon

11. Discussion Assign each planet to an hour of the day: Saturn 1, Jupiter 2, Mars 3, Sun 4, Venus 5, Mercury 6, Moon 7, Saturn 8, Jupiter 9, … What is the first hour of each day of the week?

12. Discussion How do we estimate how far away things are from us?

13. Parallax

14. Discussion Does this work with astronomical objects, like the stars, the Sun, the Moon and the planets? Can you tell by looking at them which is farther from you than the others?

16. Discussion At any given time we can only see at most half of the celestial sphere. How come?

18. Diurnal (daily) motion of the Sun The Sun rises in the east and sets in the west. Thus, each day it crosses the meridian. AM – ante meridian PM – post meridian When the Sun crosses the meridian it is also the farthest from the horizon.

20. Discussion Can you tell by observing the Moon that it is in fact closer to you than the stars? If so, how?

21. Occultation of Regulus

22. Occultation of Saturn

24. Discussion Why do you think the Moon looks much bigger near the horizon than when it is high in the sky?

27. The celestial poles and equator north celestial pole – point in the sky directly above the Earth’s north pole south celestial pole – point in sky directly above the Earth’s south pole. celestial equator – midway between the celestial poles (90 degrees away), lies directly above the Earth’s equator.

30. Circumpolar stars Not all stars rise and set. Some stars, called circumpolar, always appear above the horizon.

31. Circumpolar stars

32. Fake star trail picture

33. All the stars appear to circle a point in the sky called the celestial pole. In the north this point lies near the star Polaris, the north star. In the south this point lies near the Southern Cross.

37. Star trails on the celestial equator

38. Discussion Where on Earth would all visible stars be circumpolar?

39. Discussion If we were at the North pole, how far above the horizon would the North celestial pole be?

40. Discussion Where on Earth would none of the visible stars be circumpolar?

41. Discussion How high above the north horizon is the north celestial pole from Earth’s equator?

43. Discussion How high above the northern horizon would the north celestial pole appear in Cleveland which has a latitude of about 42 degrees?

44. Measuring latitude No matter where on Earth you are, you can easily determine your latitude by measuring the angle of the celestial pole from the horizon.

45. Positions in the sky If we imagine the sky as the inside surface of a sphere, we can specify any point on this celestial sphere using two coordinates.

46. Discussion On Earth’s surface, also a sphere, we specify positions using latitude and longitude. How does this system work?

48. Celestial coordinates The angle from the celestial equator is called the declination and the angle from the celestial “prime meridian” is called the right ascension. The celestial equator has a declination of 0 degrees, while the north celestial pole has a declination of 90 degrees. The RA is measured in hours where 360 degrees = 24 hours.

50. Earth’s rotation The diurnal motion is due to Earth’s rotation The stars take 23 hours 56 minutes from one meridian crossing to the next, this is Earth’s actual rotation period called the sidereal day