Navigating the Night Sky Theory

Pinpointing a Position How do we define the position of a point on the surface of the Earth? What are the two sets of lines called? Find the position of your home or school using this system: Make a note of this – it will help you when planning astronomy observing sessions

Mapping The Sky (1) Now, imagine the Earth as a glass ball with the lines of latitude and longitude marked on it Imagine too that you were standing at the centre of the Earth, looking out You would see the sky background with a grid of lines drawn across it This grid could be used to pinpoint places on the sky background

Mapping The Sky (2) Astronomers call the Background sky the CELESTIAL SPHERE What is the line of zero latitude on Earth called? The loop around the sky parallel to this line is called the CELESTIAL EQUATOR Look at the diagram – find out what the ‘ECLIPTIC’ is and why it is at an angle to the celestial equator

DECLINATION Declination is the astronomy equivalent of latitude i.e.: Declination is the angle in degrees of a point in the sky above or below the celestial equator What object lies at 90 o above the celestial equator?

RIGHT ASCENSION(1) To pinpoint a position on the sky we need TWO co-ordinates (just like Latitude and Longitude on Earth) The astronomy equivalent of longitude is called RIGHT ASCENSION Just like longitude, right ascension is slightly more tricky to define than latitude or declination (Defining longitude was a serious problem 300 years ago – read ‘Longitude’ by Dava Sobel to find out why!)

RIGHT ASCENSION (2) To define longitude we need a reference line to start from, which we call the PRIME MERIDIAN Which reference point on Earth does the prime meridian run through (think about its alternative name) On the celestial sphere, the reference line is called the CELESTIAL MERIDIAN The right ascension is measured from the celestial meridian BUT….

RIGHT ASCENSION (3) …Right Ascension is not measured in degrees but in HOURS, MINUTES and SECONDS!! Why? What does the sky appear to do when we look out at it from the Earth? EXERCISE: –How long does it take the Earth to rotate once? –How many degrees is one rotation? –How many degrees does it rotate per hour? –So, if an object has an RA of one hour, how many degrees is it from the celestial meridian? Reference:

ASCENSION EXTENSION – Read this if you want to! The zero line of right ascension is defined as the point in space behind the centre of the Sun when it crosses the celestial equator at midday on the spring equinox

Exercise Open the Stellarium programme (Ask if you don’t know how to use it) Adjust the time to midnight tonight Apply the ‘Equatorial Grid’ Using the ‘Search Window’ find the following stars and make a table of their co-ordinates in RA and DEC: –Polaris; Arcturus; Capella; Dubhe; Mizar

Polaris – the most useful star in the sky? Being able to find Polaris will not only show you where North is when you are lost at night, but can also help you find your latitude – here’s how (but you need a little bit of geometry)

EXERCISE Open Stellarium Apply the Equatorial grid Adjust the time to midnight Set the location for Cardiff (Lat 51.5N; Long 3.2W) Drag the screen round to point North Use the grid to measure the difference in declination between the horizon and Polaris Compare this with the latitude of Cardiff given above!

Circumpolar Stars(1) In this picture why is the tree stationary but the stars blurred out into curves? Write an explanation of: 1.How the camera was set up to take this picture 2.Why the stars make curved tracks More tricky: 1.How long the camera shutter was open for 2.Why curves near the centre are shorter than those near the edge Ask someone to check your explanations

Circumpolar Stars (2) Circumpolar stars are ones which are ALWAYS above the horizon from a particular latitude EXERCISE: –Open Stellarium for Cardiff –Apply the Equatorial Grid –Centre on Polaris –Use PAGE DOWN to zoom out until you can see the horizon all around –Use fast forward to make the sky rotate more quickly –Looking at the grid, can you work out which part of the sky is ALWAYS above the horizon? –Work out the rule for the DECLINATION of a CIRCUMPOLAR STAR at any latitude (ASK for help if you get stuck!)

TEST YOURSELF What is the definition of DECLINATION? What is the definition of RIGHT ASCENSION? How many degrees does the Earth turn in one hour? How many degrees above the northern horizon is Polaris from Cardiff? What will be the smallest possible declination of a circumpolar star from Cardiff?

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