Introduction to Positional Astronomy

The Night Sky

Nick Devereux 2006

Nick Devereux 2006

Nick Devereux 2006

Diurnal Motion

(nightly rising andsetting of stars causedby 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 365days 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 hourswhich 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 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 isconstantly 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.

The Celestial Co-ordinate System

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 are circumpolar.

For Prescott, = 34o, so all stars with declinations are circumpolar.

Nick Devereux 2006