The Local Sky

• Appearance: The horizon and sky dome (half sphere); The zenith and the meridian.
• Locating objects: By altitude and azimuth (from N, along the horizon); or by constellation.
• Sizes and distances? Only angular ones, in degrees (Sun or Moon = 0.5°), arcminutes (1' = 1°/60 = a dime at 60 m), arcseconds (1" = 1'/60 = a dime 2 miles away).
• The daytime sky: Why is the sky blue? Is it blue in space or on the Moon? Are there stars and planets in the sky during the day?
• At night: For good observation, avoid (and try to prevent!) light pollution; the best places are high and dry. If possible, use binoculars.
• The sky today: Familiarize yourself with the objects that are visible by checking websites, and/or running planetarium software.

Objects in the Sky

• Most obvious: The Sun, Moon, stars, planets, ... the Milky Way; The view changes with location and time, and there are parts of the sky (like Centaurus) we can never see from here! How can we tell planets and stars apart with the naked eye? Planets move over time, and stars often "twinkle".
• Constellations: 88 patterns are now the official ones (including 12 in the Zodiac), but more have been at times recognized (they also change); Stars in one constellation are usually not really close to each other, they just appear to be.
• Other objects: Some are temporary, like comets, asteroids, meteors and fireballs, auroras,... spacecraft, exploding stars, junk...! Some are fuzzy, like comets, nebulae, star clusters, galaxies. The brightest parts of a few galaxies can be seen with the naked eye; Most cannot because they are too faint, not too small (a nebula can cover 3°, the Virgo cluster covers 15°!).
• Brightness: Magnitudes, in order of decreasing brightness, 1-6 for the naked eye in ideal conditions; This includes about 8000 stars (the brightest is Sirius), and 5-6 planets.
• Things we don't see: A constant flux of particles from the Sun and cosmic rays, light (of course) and other forms of radiation.