1. Astronomy Foundations – Chapter 0

2. 0.1 The “Obvious” View The celestial sphere:
Stars seem to be on the inner surface of a sphere surrounding the Earth.
They aren’t, but we can use two-dimensional spherical coordinates (similar to latitude and longitude) to locate sky objects.
Sky is divided into 88 constellations

3. 0.1 The “Obvious” View
Declination: Degrees north or south of celestial equator
Right ascension: Measured in hours, minutes, and seconds eastward from position of the Sun at vernal equinox

4. 0.2 Earth’s Orbital Motion
Daily cycle, noon to noon, is diurnal motion – solar day.
Stars aren’t in quite the same place 24 hours later, though, due to Earth’s rotation around the Sun; when they are in the same place again, one sidereal day has passed.

5. 0.2 Earth’s Orbital Motion
The 12 constellations the Sun moves through during the year are called the zodiac; path is ecliptic.

6. 0.2 Earth’s Orbital Motion
Ecliptic is plane of Earth’s path around the Sun; at 23.5° to celestial equator.
Northernmost point (above celestial equator) is summer solstice; southernmost is winter solstice; points where path crosses celestial equator are vernal and autumnal equinoxes.
Combination of day length and sunlight angle gives seasons.
Time from one vernal equinox to next is tropical year.

7. The Earth's rotation axis is tilted with respect to its orbit around the Sun => seasons.
Summer
Winter
Sun low in northern sky
Sun high in northern sky
Scorpius
Day
Night
Orion
Night
Day
Tilt is 23.5o

8. Summer
Winter
In winter, sunlight is spread out more thinly across the ground => each bit of ground receives less radiation => cooler

9. The Year
Summer
Winter
Day
Night
Orion
Scorpius
Night
Day
The Earth revolves around the Sun in days (“sidereal year”). But the year we use is days (“tropical year”). Why?

10. Precession Period 26,000 years!
The Earth has a bulge. The Moon "pulls down" on the side of the bulge closest to it, causing the Earth to wobble on its axis (how do we know this?)
Earth
Moon
Vega * *
Polaris
Spin axis
Precession Period 26,000 years!

11. Now 13,000 years from now Summer: July Winter: January
Scorpius
Night
Day
Day
Night
Orion
Summer: July
Winter: January
13,000 years from now
Scorpius
Night
Day
Night
Orion
Day
Winter: July or January?
Summer: January or July?
We choose to keep July a summer month, but then in 13,000 years, summer occurs on other side of orbit!

12. The Motion of the Moon
The Moon has a cycle of "phases", which lasts about 29 days.
Half of the Moon's surface is lit by the Sun.
During this cycle, we see different fractions of the sunlit side.
Which way is the Sun here?

14. DEMO - Phases of the Moon
The Motion of the Moon
DEMO - Phases of the Moon

15. Cycle of phases or "synodic month" Orbit time or "sidereal month"
Cycle of phases slightly longer than time it takes Moon to do a complete orbit around Earth.
Cycle of phases or "synodic month"
Orbit time or "sidereal month"
29.5 days
27.3 days

16. Eclipses Lunar Eclipse
When the Earth passes directly between the Sun and the Moon.
Sun
Earth
Moon
Solar Eclipse
When the Moon passes directly between the Sun and the Earth.
Sun
Moon
Earth

17. Solar Eclipses Diamond ring effect - just before or after total Total
Partial
Annular - why do these occur?

18. Lunar Eclipse

19. Top view, exaggerated ellipse
Moon's orbit tilted compared to Earth-Sun orbital plane:
Sun
Moon
Earth
5.2o
Side view
Moon's orbit slightly elliptical:
Moon
Distance varies by ~14%
Earth
Top view, exaggerated ellipse

20. Types of Solar Eclipses Explained

22. Clicker Question:
Why is it warmer in Albuquerque in the summer than winter?
A: The northern hemisphere is tilted towards the sun in summer.
B: The Earth is closer to the sun in summer.
C: The greenhouse effect increases in summer.
D: The sun increases its intrinsic luminosity in the summer.
E. All of the above.

23. Clicker Question: Have you ever seen a solar eclipse?
A: Total eclipse of the sun.
B: Partial solar eclipse.
C: None
Note: Total solar eclipse on Jan 4, 2011

24. Recent and upcoming total and annular solar eclipses

25. Clicker Question: Have you seen a lunar eclipse?
A: Total eclipse of the moon.
B: Partial lunar eclipse.
C: None
December 21, 2010

26. Why don't we get eclipses every month?
A: The moon has lots of holes in it.
B: The moon moves too far away to block the sunlight.
C: The orbit of the moon is tilted.
D: We do get them every month but don’t notice.

27. The "Solar Day" and the "Sidereal Day"
How long it takes for the Sun to return to the same position in the sky (24 hours).
Sidereal Day
How long it takes for the Earth to rotate 360o on its axis.
These are not the same!

28. One solar day later, the Earth has rotated slightly more than 360o .
A solar day is longer than a sidereal day by 3.9 minutes
(24 hours vs hours 56 minutes seconds).

29. Eratosthenes Determines the Size of the Earth in about 200 B.C.
Sun's rays
Syene
Alexandria N S
7.2o
Earth

30. He knows the distance between the two cities is 5000 "stadia".
From geometry then,
7.2o
5000 stadia =
360o
Earth's circumference
=> circumference is 250,000 stadia, or 40,000 km.
So radius is:
40,000 km
6366 km = 2p
(very close to modern value, 6378 km!)

31. Pluto: Planet or Plutoid
On March 13, 2007 – New Mexico proclaimed Pluto Planet Day at the legislature
New Mexico State University and Dona Ana county were the longtime home of Clyde Tombaugh, discoverer of Pluto
He was 24 when he discovered Pluto using a specially built 13” telescope at the Lowell Observatory in Arizona (1930)
March 13th is the birthday of Percival Lowell and on March 13, 1930, the Lowell Observatory announced the discovery (It took time for confirmation from other observatories)