Astronomy Astrophysics

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).

Celestial Sphere

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

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.

Extended Light Source Shadows ROT Umbra Penumbra

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

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

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?

Lunar Phases

Man in the Moon

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.

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

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 700 000 parsecs (pc) away. Why don’t we use km?

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 0.0006 ly away.

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

Milky Way

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