Lecture 5 1/21/2013 12:24:00 PM
Cycles of the Sky, Part I
Reading: Chapter 2.
Quasar: a type of extremely bright galactic core containing a super-massive
At what time of year is the Earth closest to the sun?
The earth is closest to the sun at the beginning of January.
Our winter is at the same time as the Southern Hemisphere’s
summer, and vice versa.
So. Any explanation of the seasons must explain why they are
opposite in the northern and southern hemispheres, and cant
depend only on our distance from the sun.
Declination and right ascension are the equivalent of latitude and
longitude for the celestial sphere:
o Declination: angular distance north or south from the
celestial equator (negative if south)
o Right ascension: an angle measure east from the vernal
equinox, usually measured in hours.
A few terms describing the local sky wherever you are standing:
o Zenith: the point directly above your head:
Meridian: a north-south line passing through the zenith.
Sometimes also called the celestial meridian.
Horizon: all points at 90˚ away from the zenith. (where
the ground meets the sky)
Other occasionally useful terms:
o On the surface of any sphere, a circle whose centre is the
same as the centre of the sphere (for example, the equator is
a great circle, but other latitude lines are not.)
Hour circle: o Any great circle passing through both celestial poles—or, a
line of right ascension, equivalent to longitude lines on Earth.
A star that never sets, but always stays above the horizon.
Which starts are circumpolar depends on your latitude
o At the north pole, all visible starts are circumpolar.
o At the equator, none are circumpolar.
How do we figure out which stars re circumpolar?
Suppose your latitude is 40˚N.
Then the north celestial pole is 40˚ above the horizon.
Then any star less than 40˚ away from the NCP can never be below
That means, circumpolar stars have declinations bigger than 90
minus whatever your latitude is. (90 – 40 = 50˚)
More generally, circumpolar stars have declinations bigger than 90