The Local Sky An object’s altitude (above horizon) and direction (along horizon) specifies its location in your local sky

The Local Sky

Zenith: The point directly overhead

Horizon: All points 90° away from zenith

Meridian: Line passing through zenith and connecting N and S points on horizon

Is the zenith the same as the north celestial pole ?

1 Specifying Locations on Earth

• When we describe the sky, we must specify where on earth we are observing the sky from. • We specify our location on earth using latitude and longitude .

Specifying Locations on Earth • Imagine the Earth as a transparent sphere (actually the because of Earth's rotation, its equator bulges out a little). • Through the transparent Earth we can see its equatorial plane. The point O is the center of the Earth. • The angular elevation of a point P north or south of the equatorial plane is its latitude. • All points of equal elevation lie on the same circle of latitude.

2 Specifying Locations on Earth • Meridians of longitude are semicircles going through the north pole, south pole and crossing the equator. • The meridian passing through Greenwich, England is historically chosen to be 0 degrees. It is called the Prime Meridian. • The other longitudes are designated by the angle east or west of the equator where they cross the equator.

Specifying Locations on Earth • The longitude of a point is determined by the meridian passing through it. • All points with longitude between 0 and 180 degrees west of the equator lie in the Western Hemisphere • All points with longitude between 0 and 180 degrees east of the equator lie in the Eastern Hemisphere

Laser marking Prime Meridian in London

3 Specifying Locations on Earth

• Latitude: position on Earth north or south of equator • Longitude: position on Earth east or west of prime meridian (runs through Greenwich, England)

What is the latitude and longitude of Waterloo?

Waterloo: 43.5 N 80.5 W

Go to www.fourmilab.ch/cgi-bin/Earth

to view this location from space

Specifying Locations on Earth

• Riddle: An astronomer travels 5 degrees of latitude south, 5 degrees of longitude east, 5 degrees of latitude north and returns to her starting point, where she sees a bear. What colour was the bear ?

4 Specifying Locations on Earth • Longitude helps us determine universal and local time. • The earth rotates through 360 degrees of longitude in 24 hours. Thus the earth rotates through 15 degrees of longitude in 1 hour. • As the Earth rotates around its axis, at any moment one line of longitude--"the noon meridian"--faces the Sun, and at that moment, it will be noon everywhere on it.

Specifying Locations on Earth • Longitude helps us determine universal and local time. • Thus when it is noon in Greenwich it is 11:00 at all points at longitude 15 degrees W and 13:00 at all points 15 degrees E. (What happens at the 180 degrees W/E longitude ?) • Astronomers use Greenwich mean time (time at the Prime Meridian) as the universal time to record events.

5 Stellar Coordinates

• Declination: stellar ‘latitude’ • Right Ascension: stellar ‘longitude’

Variation of the sky with position on earth Question The North Star (Polaris) is 50° above your horizon, due north. Where are you? A. You are on the equator. B. You are at the North Pole. C. You are at latitude 50°N. D. You are at longitude 50°E. E. You are at latitude 50°N and longitude 50°E.

6 Variation of the sky with position on earth Altitude of the celestial pole in your sky = your latitude

Since the North Star (Polaris) and the Southern Cross help to identifies the celestial poles, they are very useful for navigation

Daily Motion of Objects in the Sky

Stars rise and set every night Earth rotates from west to east so stars appear to circle from east to west.

7 Our view of daily motion of the stars • Stars near the north celestial pole are circumpolar and never set for observers in the Northern hemisphere (all latitudes north of the equator). • In the Northern hemisphere we cannot see stars near the south celestial pole.

Variation of the sky with position on earth The night sky varies with latitude but not longitude.

• The night sky depends on latitude because your position on Earth determines which constellations remain below the horizon.

8 Question What is the arrow pointing to? A. the zenith B. the north celestial pole C. the celestial equator

Question What is the arrow pointing to? A. the zenith B. the north celestial pole C. the celestial equator

9 Yearly motion of constellations Constellations we see depend on time of year:

They depend on time of year because Earth’s orbit changes the apparent location of the Sun among the stars.

Why do we see the same constellations as ancient astronomers thousands of years ago ?

Precession of Earth’s axis • Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years. • The gravity of the Sun and Moon cause the Earth’s axis to precess in a slow circular sweep.

Earth’s axis precesses like the axis of a spinning top

10 Precession of Earth’s axis

• Hipparchus discovered the precession of the earth’s axis over 2000 years ago ! • Precession may affect the severity of the different seasons and may cause extreme climate changes such as ice ages

Earth’s axis precesses like the axis of a spinning top

Precession of Earth’s axis – In about 12,000 years, the pole will have moved to the apparent vicinity of the very bright star Vega (alpha Lyrae). – The figure shows the apparent path followed by the north celestial pole over thousands of years.

11 Observing the skies for yourself • Find a dark spot far from light pollution of cities and cars • Use a sky chart (star map). There are some in your text. • Turn the chart until the horizon you are facing is at the bottom. • The stars on the bottom of the chart are now directly in front of you • The centre of the chart shows the overhead sky • The edges show the horizon around you • A pair of binoculars will be helpful to pick out faint objects • Try to find the brighter constellations and stars first http://media.skyandtelescope.com/documents/GettingStartedNorth.pdf

Sky Chart: Northern Hemisphere

12 Sky Info: Northern Hemisphere More resources http://www.fourmilab.ch/yoursky/ http://www.planetarium.montreal.qc.ca/Information/ciel_du_mois_a.html http://maps.seds.org/Const/constS.html http://www.nrc-cnrc.gc.ca/eng/education/astronomy/constellations/index.html http://hubblesite.org/explore_astronomy/tonights_sky/ http://www.skyandtelescope.com/observing/ataglance

Summary • What does the universe look like from Earth? – We can see over 2,000 stars and the Milky Way with our naked eyes, – A map of the sky is divided into 88 regions or constellations. – Many tar names have ancient Latin, Greek or Arabic roots – The celestial sphere is a convenient tool for describing the sky – We can specify the position of an object in the local sky by its altitude above the horizon and its direction along the horizon – We can measure a stars angular size but not it actual size with our naked eye.

13 Summary • What does the universe look like from Earth? – The brightness of stars is measured using the apparent visual magnitude scale originally used by Hipparchus and Ptolemy – Modern instruments measure the flux: the total light energy hitting one square metre per second – Flux can then be used to calculate apparent visual magnitude – Some stars are so bright they have negative magnitudes – Faint stars detected by telescopes have magnitudes larger than 6 – Apparent visual magnitude is based only on visible light. – Apparent visual magnitude does not say anything about the stars actual energy output since their distance is not included.

Summary

• What is the daily motion of stars as seen from Earth? – Stars rise in the east and set in the west because of the Earth’s rotation – The sky varies with latitude but not longitude – Local time of astronomical events varies with longitude – Universal time is taken to be Greenwich mean time • What is the daily motion of stars as seen from Earth? – The constellations depend on time of year because Earth’s orbit changes the apparent location of the Sun among the stars.

14 Specifying Locations in the sky

• Declination (DEC) and Right Ascension (RA) are extensions of latitude and longitude to the celestial sphere. • Declination is measured in positive or negative degrees going north or south of the celestial equator • RA is measured in hours which increase going east. • 0 hrs corresponds to the crossing of the ecliptic and the celestial equator

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