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Introduction to Positional
Astronomy
The Night Sky
 Nick Devereux 2006
 Nick Devereux 2006
 Nick Devereux 2006
Diurnal Motion
(nightly rising and
setting of stars caused
by earths rotation )
 Nick Devereux 2006
The Elevation of the North Pole Star
The north pole star is always at an elevation, or altitude, a, above the
northern horizon, that is equal to the latitude, of the observer.
Circumpolar stars are stars which are always in view.
They never set below the horizon.
For Prescott,  = 34o.
 Nick Devereux 2006
Annual Motion is caused by the Earths orbit around the
Sun. The Earth makes one orbit (360o) every year or 365
days which is slightly less than 1 degree per day actually 0.98
degrees/day.
Now, the Earth also spins on it’s axis 360 degrees in 24 hours
which works out to be 15 degrees per hour, or 15 degrees/60 min
= 0.25 degrees/minute.
So, if you want to see the stars in exactly the same place in the
sky as the night before, you have to go out 0.98/0.25 = 3.9
which is ~ 4 minutes earlier than the previous night!
The Celestial Co-ordinate System
The Celestial co-ordinate system is analogous to latitude and longitude.
Declination is analogous latitude, as both are measured with respect to
the equator.
The celestial equivalent of Longitude is a bit more complicated
because the Earth is turning on it’s axis, and moving about the Sun,
so the projection of the Earth’s longitude system on the night sky is
constantly moving.
Astronomers have thus adopted a specific location on the sky, known
as the “First Point in Aries”, that is actually the intersection of the
ecliptic and the celestial equator, (where the Sun is on the Spring
equinox), to mark the zero point of the celestial longitude system,
called Right Ascension.
Right Ascension& Declination
 Nick Devereux 2006
The Elevation of the North Pole Star
The north pole star is always at an elevation, or altitude, a, above the
northern horizon, that is equal to the latitude, of the observer.
Circumpolar stars are stars which are always in view.
They never set below the horizon.
All stars with declinations 90-  are circumpolar.
For Prescott,  = 34o, so all stars with declinations 90-=56
are circumpolar.
 Nick Devereux 2006