1. Groups of dark brown streaks have been photographed by the Mars Reconnaissance Orbiter on melting pinkish sand dunes covered with light frost. Dark sand on the interior of Martian sand dunes near the north pole of Mars became more and more visible as the spring Sun melts the lighter carbon dioxide ice. When occurring near the top of a dune, dark sand may cascade down the dune leaving dark surface streaks. Objects about 25 centimeters across are resolved on this image spanning about one kilometer.

2. Homework #1 has been posted.
Due: Thursday, January 28, 4:00 pm.
Be sure to have your answers determined before accessing the homework on Oncourse.

3. How do you find things on a sphere?
Celestial Sphere:
How do you find things on a sphere?

4. On the Celestial sphere any location can be given by its:
Declination (Celestial Latitude, measured north or south of the Celestial equator)
Right Ascension (Celestial Longitude, measured (angular distance from the “Vernal Equinox”)

5. The celestial sphere is “fixed” in space.
We view it in the context of the local sky

7. We have a sphere (the Earth) within a sphere (Celestial sphere)
We have a sphere (the Earth) within a sphere (Celestial sphere). If both were fixed and unchanging, we would always look to the same point in the sky to see the same object. BUT…
We can be located anywhere on the Earth
The Earth is rotating
The Earth is orbiting the Sun
The Moon is orbiting the Earth
The planets are also orbiting the sun
We need to take all of this into account to understand changes in the sky, time, seasons, etc.

8. Half of the Celestial Sphere is visible in the local sky, the other half is below the horizon.
Rotation of the Earth causes the portion of the celestial sphere visible in the local sky to change with time.

9. The Earth rotates west to east .
This causes the (fixed) celestial sphere to appear to rotate east to west
The apparent motion of the stars is referred to as “diurnal motion”.

10. Diurnal circle
Diurnal motion leads stars to “move” around the celestial sphere at a constant angular distance from the celestial pole.

11. Half of the celestial sphere is always visible above the horizon.
As you move around on the Earth, the portion of the celestial sphere visible in your local sky changes.
Half of the celestial sphere is always visible above the horizon.
Moving north or south (changing your latitude), leads to viewing a different portion of the celestial sphere.

14. How does the altitude of the North Celestial Pole change with the latitude of an observer?
(hint: imagine observer’s at the North Pole, at the Equator, and at a mid-latitude location, such as Bloomington).

15. Location: Fairbanks, AL
(Figures from Nick Strobel’s “AstronomyNotes” website -

16. Location: Seattle, WA

17. Location: Los Angeles

18. Location: Equator

19. IMPORTANT POINTS!!
1. The altitude of the celestial pole is equal to the latitude of the observer
2. The Celestial Equator ALWAYS intersects the East and West points on the horizon.

21. Depending upon your location on Earth, some stars will either never set (i.e., they are always above the horizon) and some stars will never rise (they are always below the horizon). These stars are called “Circumpolar Stars”

22. Circumpolar Stars: never set

23. The star named “Polaris” is very near the North Celestial Pole (within one degree).
It is also referred to as the “North Star”.

24. What is the “Meridian”?
The longitude passing through Greenwich England
The point directly over an observer’s head
A great circle half way between the observer’s zenith and nadir
An imaginary line in the sky that passes from the north point on horizon, through the zenith, to the south point on horizon

25. What is the “Meridian”?
The longitude passing through Greenwich England
The point directly over an observer’s head
A great circle half way between the observer’s zenith and nadir
An imaginary line in the sky that passes from the north point on horizon, through the zenith, to the south point on horizon

26. Stars which are not circumpolar:
rise in the eastern half of the sky (i.e., east of the meridian),
increase their altitude until they cross the meridian ,
set in the western half of the sky (i.e., west of the meridian).

27. ConceptTest
You are in Bloomington and observe a star rising directly to the east. When this star reaches its highest point above the horizon, where will it be?
(a) high in the northern sky
(b) high in the eastern sky
(c) high in the southern sky
(d) high in the western sky
(e) at the zenith

28. ConceptTest
You are in Bloomington and observe a star rising directly to the east. When this star reaches its highest point above the horizon, where will it be?
(a) high in the northern sky
(b) high in the eastern sky
(c) high in the southern sky
(d) high in the western sky
(e) at the zenith

29. ConceptTest
Where are circumpolar stars at their lowest point in the sky?
In the western sky
In the eastern sky
In the southern sky
On the meridian

30. ConceptTest
Where are circumpolar stars at their lowest point in the sky?
In the western sky
In the eastern sky
In the southern sky
On the meridian

31. In Bloomington, where is the North Celestial Pole?
On the Celestial Equator
On the meridian at an altitude of approximately degrees.
Near the zenith
At the north point on the horizon

32. In Bloomington, where is the North Celestial Pole?
On the Celestial Equator
On the meridian at an altitude of approximately degrees.
Near the zenith
At the north point on the horizon

33. You are in Bloomington and observe a star rising directly to the east
You are in Bloomington and observe a star rising directly to the east. Where does this star lie on the Celestial Sphere?

34. ConceptTest
As you move from the Earth's equator toward the North Pole, the number of circumpolar stars:
increases
decreases
stays the same

35. ConceptTest
As you move from the Earth's equator toward the North Pole, the number of circumpolar stars:
increases
b. decreases
c. stays the same

36. What about the Sun’s motion?
We have been examining the daily motion of the stars through the local sky.
What about the Sun’s motion?

37. Got here

38. The Sun moves east to west, full circuit around the sky, each “day” (result of Earth’s rotation)
Relative to the stars, the Sun moves slowly eastward each day (~ 1 degree/day): full circuit around Celestial Sphere in one “year” (result of Earth’s orbit)

39. The Sun moves east to west, full circuit around the sky, each “day” (result of Earth’s rotation)
Relative to the stars, the Sun moves slowly eastward each day (~ 1 degree/day): full circuit around Celestial Sphere in one “year” (result of Earth’s orbit)
Path of Sun on Celestial Sphere due to Earth’s orbital motion is a great circle called the “Ecliptic”

40. The Sun moves east to west, full circuit around the sky, each “day” (result of Earth’s rotation)
Relative to the stars, the Sun moves slowly eastward each day (~ 1 degree/day): full circuit around Celestial Sphere in one “year” (result of Earth’s orbit)
Path of Sun on Celestial Sphere due to Earth’s orbital motion is a great circle called the “Ecliptic”
Constellations the Ecliptic passes through are referred to as the “zodiac”

41. Two measures of the Earth’s rotational period
Solar day: Rotational period relative to the sun (successive sun crossings of the meridian). This is the common measure of the day.
Sidereal Day: Rotational period relative to the stars (successive star crossings of the meridian, 23h 56m)

42. “angular distances” Full circle = 360 degrees Right angle = 90 degrees
Fist at arm’s length ~ 10 degrees
Moon & sun ~ ½ degree wide
1 Hour = 15 degrees
1 minute = 1/60 hour = 1/4 degree

43. Definitions
ecliptic plane = The plane of the Earth’s orbit
ecliptic = The annual path of the Sun on the sky
equatorial plane = The plane of the Earth’s equator
celestial equator = Projection of the equatorial plane onto the sky

44. The Earth orbit is prograde, i. e
The Earth orbit is prograde, i.e., it is in the same direction as its rotation.
Thus, the Earth’s orbital motion causes the sun to appear to move eastward (increasing R.A.) along the ecliptic around the celestial sphere.

45. Today we divide the celestial sphere into 88 regions, referred to as
Today we divide the celestial sphere into 88 regions, referred to as constellations (in a manner similar to dividing the Earth’s surface into countries).

46. The constellations along the ecliptic are called the zodiac (see applet)

47. The Ecliptic & Celestial Equator are Great Circles that:
are inclined by 23.5 degrees to each other
intersect with at two points
One of these points of intersection is where the sun is located on the celestial sphere at the moment Spring begins. This point is referred to as the “Vernal Equinox”

48. The Vernal Equinox serves as the origin for measuring Right Ascension (R.A. = celestial longitude)

49. Right Ascension - R.A. (Celestial Longitude)
Angular distance east of the Vernal Equinox
Measured in “time” units (1 hour = 15o)

50. Winter Solstice – sun at farthest point south
Vernal (Spring) Equinox – Sun on Celestial equator moving north
Summer Solstice – sun at farthest point north
Autumnal (Fall) Equinox – Sun on Celestial equator moving south