AST 101
What Astronomers Measure
Where is it?
"What good are Mercator's North Poles and Equators
Tropics, Zones, and Meridian Lines?"
So the Bellman would cry, and the crew would reply
"They are merely conventional signs"
L. Carroll -- The Hunting of the Snark
The sky appears as a 2-dimensional sphere
• Celestial Coordinates
• Ecliptic Coordinates
• Galactic Coordinates
• Alt-Az Coordinates
When is it?
The times of your lives…
Time keeping is tied to the Earth and the Sun
• Local solar time
• Civil time
• Universal time
• Sidereal time
Second: the duration of 9,192,631,770 periods of the radiation
corresponding to the transition between two hyperfine levels of the
ground state of 133Cs
Day: 86,400 seconds
Julian day: days elapsed since 12UT on 4713BC 1 January
How Bright is it?
• Photometry
• Spectroscopy