1. Astronomy
Astrophysics

2. The Celestial Sphere
The Earth rotates about its axis and revolves around the sun.
We sometimes talk as though the Earth were fixed and the celestial objects (stars, sun, moon, planets) move around us.
This is based on antiquity and is not true but it is sometimes a good way to look at the situation.
Imagine the stars are attached to the inside of a giant black sphere with the Earth at its centre.
The NCP is the North Celestial Pole and is roughly where we see Polaris (North star).

3. Celestial Sphere

4. Point Light Source
Eclipses are the business of shadows – a point source of light behind an object will cast a diverging shadow of light called the umbra.
Umbra

5. Extended Light Source
With non-point sources of light, like the sun, the shadow cast by the body has three regions.
A dark converging conical shadow – umbra, no light from source seen.
On both sides of the umbra, semi-dark diverging regions – penumbra, some light seen from source.
Extending the lines of the umbra is the ROT (region of transit) where the light source appears like a donut.

6. Extended Light Source Shadows
ROT
Umbra
Penumbra

7. Solar eclipses
A solar eclipse occurs when the moon is between the sun and the Earth.

8. Lunar eclipse
A lunar eclipse occurs when the moon passes behind the Earth such that the Earth is between the sun and the moon.

9. Lunar Motions
We always see one face of the moon (The man in the moon).
What has to occur for this to happen?
Does the moon rotate as it orbits Earth?

11. Lunar Phases

12. Man in the Moon

13. Seasons The Earth is rotating on its own axis as it orbits the sun.
The axis is tilted at 23.5 degrees from the vertical and this is the main cause of the seasons as different parts of the planet get varying amounts of solar radiation as we move.
The Earth is in an elliptical orbit so there are times when we are farther from the sun, but these changes are too small to affect the Earth and thus its seasons.

14. Earth Solar orbit
Autumnal equinox September 23 Northern fall, Sun over equator
23.5O
Axis of rotation
Winter solstice December 21 northern winter, Sun over Tropic of Capricorn
Summer Solstice June 21 northern summer Sun over Tropic of Cancer
SUN
Direction of revolution
Vernal equinox March 21 Northern spring, Sun over equator

15. How far to the Stars?
The sun is 1 astronomical unit (au) from the Earth.
The nearest star, Proxima Centauri is 4.1 light years (ly) away.
The nearest galaxy, Andromeda is parsecs (pc) away.
Why don’t we use km?

16. How far to the Stars?
1 AU is 150 million kilometres. These units are used for our solar system.
1 ly is the distance travelled by light in a year (it is NOT a time). Light travels at 300 million metres every second!
A pc is 3.26 ly. Megaparsecs are used for distances between galaxies.
The Earth-moon distance is 1.28 light seconds and the Earth-sun is 8.3 light minutes.
Pluto is ly away.

17. Milky Way
Our galaxy is ~100,000 ly in diameter and has a thickness of ~ 2000 ly.
It has a bulging central nucleus and spiral arms and our sun is on one of these arms greater then halfway from the hub ~28000 ly.
Our galaxy is estimated to contain 1011stars and our sun orbits the hub once every 200 million years

18. Milky Way

19. Class/Home work Page 308 #1, 3-9 Page 328 #1, 3, 5, 7, 8, 10, 11, 13