Introduction to the Sky

  The Local Sky

  • Appearance: The horizon and sky dome (half sphere); The zenith and the meridian.
  • Locating objects: By altitude and azimuth (or direction, from N along the horizon).
  • Variation: What you see depends on your location, especially latitude (there are parts of the sky, like Centaurus, that we can never see from here!), and there is a constant apparent motion over time because of the Earth's rotation; One way to solve this problem is to locate objects by constellations.
  • 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? Because light from the Sun is scattered by the atmosphere. (Are stars and planets up during the day too?)
  • The nighttime sky: For good observation, avoid (and try to prevent!) light pollution; The best places are high and dry; For some observation purposes, binoculars are better than telescopes!
  • The sky today: Which objects are visible? Familiarize yourself with the sky by checking websites, and/or running planetarium software, like SkyGazer.

Objects in the Sky

  • The Moon: After the Sun, the brightest object by far (it always shows the same side to us, as we can tell from its features).
  • Stars: We can see about 8000 (of magnitude 1–6 in order of decreasing brightness) with the naked eye in ideal conditions, many more with binoculars or a telescope; the brightest one is Sirius.
  • Star patterns: Currently 88 official constellations (including 12 in the Zodiac), but they change and more were used earlier; Usually, in a constellation stars only appear to be close to each other; Some patterns are asterisms, not constellations.
  • Planets: How can we tell planets and stars apart with the naked eye? (A telescope can make it easier.) Planets move over time, and stars sometimes "twinkle". We can see 5-6 naked-eye planets.
  • Temporary objects: A continuing stream of meteors ("shooting stars"); Occasional comets, asteroids, fireballs, auroras, exploding stars, ..., and now even spacecraft and sometimes space junk.
  • Fuzzy objects: The brightest parts of a few galaxies and some star clusters can be seen with the naked eye; There are nebulae and many more galaxies; Most are way too faint to be seen, but not too small (M31 is much larger than the Moon, IC1396, an emission nebula, covers 3°, the Virgo Cluster 15°!); And then there is the Milky Way...
  • Our goal: Understand what all these objects are, how far they are, how they relate to each other, ...

In Addition to What We See...

  • Everything: Particles and radiation of all known types, including a constant flux of particles from the Sun (solar wind), interstellar wind and cosmic rays from much further away...

page by luca bombelli <bombelli at>, modified 29 sep 2012