Constellations

• One of the stellar patterns identified by name, usually of mythological origin.
• The region of the sky containing that star pattern

Asterism

• A named grouping of stars that is not one of the recognized constellations.

The Names of Stars

• Most star names are derived from ancient Arabic.
• For example: Betelgeuse—comes from the Arabic phrase yad aljawza (“armpit of Jawza”).
• Many of these ancient star names are still being used.

The Brightness of Stars

We use a magnitude scale:

• The astronomical brightness scale. Inverse logarithimic.
• 5 magnitudes $= 100x$; 1 magnitude $~ 2.512x$.

A measure of the flow of energy out of a surface of a star is called Flux.

A measure of the brightness of stars as seen by the human eye on Earth. Logarithmic: $m_v=-2.5\mathrm{log}(\frac{F}{F_0})$

Motions of Celestial Bodies

• Rotation: Motion around an axis passing through a rotating body.

• Revolution: Orbital motion about a point located outside the orbiting body.

• Precession: The slow change in Earth's axis of rotation.

Phases of the Moon

1. The Moon always keeps the same side facing Earth.
2. The changing shape of the Moon as it passes through its cycle of phases is produced by sunlight illuminating different parts of the side of the Moon seen by an observer on Earth.
3. The orbital period of the Moon around Earth is not the same as the length of a Moon phase cycle.

Eclipses

Solar Eclipses

In a solar eclipse, the Sun is completely hidden (eclipsed) by the Moon.

• Umbra: The region of the shadow that is totally shaded.
• Penumbra: The region of the shadow that is partially shaded.
• Antumbra: The region of the shadow that just ends.

Lunar Eclipses

• Darkening of the moon when it enters the Earth's shadow.
• The moon darkens and turns copper-red because of sunlight refracted through Earth's atmosphere.

Stellar Coordinates

• The celestial sphere uses a coordinate grid similar to the longitudinal and latitudinal coordinate systems used on Earth.

Stellar coordinates:

• The angular distances of the an object on the celestial sphere.
• Describes location of objects in the sky.