Constellations

·  Early civilizations divided the stars up into groups

o  Babylonians, Egyptian, Greeks, Chinese, Mongols, Norse, Native Americans, etc.

o  All assigned these groups names, and associated them with stories to make them easy to remember

o  Example: Polaris (the north star) was the:

§  Jeweled Nailhead – Norse

§  Golden Peg – Mongols

§  Emperor of Heaven – Chinese

§  Chief Star – Skidi band of the Pawnee Indians

·  Constellations of the Zodiac (circle of animals) – Figure 6-2c

o  Developed by the Babylonians

o  Originally divided the sky into 12 (Ophiuchus, the Serpent Holder is now considered one of 13) equal divisions

o  Each constellation was called a “sign” of the zodiac, 12 signs for the 12 months

o  Used for keeping time

·  Asterims – smaller familiar groupings that do not make up constellations (Example: the Big Dipper, in the constellation Ursa Major)

·  Sky now (since 1930 - IAU) divided into 88 official constellations – unequal divisions with definite boundaries (Appendix 10)

·  Naming stars

o  Greek letters – brightest to dimmest (a Orionis = Alpha of Orion = Betelgeuse); used Latin if we run out of Greek

o  Numbers – Stars position side to side in a constellation (61 Cygni)

Twinkling

·  Scintillation – stars appear to dance around slightly

·  Effect of Earth’s atmosphere

o  Starlight being bent and moved by differences in densities, temperatures, and water vapor content in the atmosphere

o  Stars – appear as 1-D point sources of light, twinkle

o  Planets / Moon – reflected sources of light (many points - 2-D), don’t twinkle, but can shimmer (like heat waves on a hot road)

Observational Differences between Planets & Stars
PLANETS / STARS
The planets move relative to stars on celestial sphere / The relative positions of the stars are fixed on celestial sphere
The nearer and larger planets appear as disks in telescope / The stars appear as "points" of light, even through the telescope
The brighter planets do not "twinkle" / The stars appear to "twinkle"
The planets are always near the imaginary yearly path of the Sun on the celestial sphere (the ecliptic) / Stars can be anywhere on the celestial sphere