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
- 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
dry; For some observation purposes, binoculars are better
- The sky today: Which objects are visible?
Familiarize yourself with the sky by checking
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
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,
- 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,
nebula, covers 3°, the Virgo
Cluster 15°!); And then there is the Milky
- 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
page by luca bombelli <bombelli at olemiss.edu>,
modified 29 sep 2012