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Lines in the Sky

• In order to use the sky to measure time you need to measure the location of objects in the sky. We will look at two methods of measuring locations in the sky.

• Both methods require measuring angles.• These methods have long been used not

only for timekeeping but for navigation as well.

• But first we need to define some terms

2

The Earth and Sky

• From where we stand Earth appears flat and the sky appears like a dome above us.

• The point where the dome of the sky meets the Earth is called the horizon.

3

The Earth and Sky• If an object is above

the horizon it is visible. If it is below the horizon it is blocked from view by the Earth

• Stars, the Sun, Moon and planets all appear to rise above the horizon in the East and set below it in the West.

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Earth’s Spin Axis• The imaginary line

connecting Earth’s North and South Poles is called the Earth’s rotation axis. The line around which the Earth spins.

• Extending this line into the sky points to the North and South Celestial Poles.

• The Celestial Equator is the extension of Earth’s Equator.

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Earth’s Spin Axis and Polaris

• Everything in the sky appears to move around the Celestial poles.

• The star Polaris lies very near the North Celestial Pole. It never rises or sets. It is always visible from the Northern Hemisphere.

• Polaris, the North Star, is never visible from the Southern Hemisphere.

Polaris, the North Star

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It is the spin of the Earth on its axis that makes the Sun and stars appear to move across the sky.

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Motion of Stars at the North Pole

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Motion of Stars at the Equator

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How to find the North Star• Polaris can be found

using the “Pointer” stars of the Big Dipper

• The Big Dipper is part of a larger constellation of stars called Ursa Major (the Great Bear)

• Now you know how to find true North.Pointer stars

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Meridian, Altitude & Zenith• Another important

point in the sky is the Zenith, the point directly overhead.

• The line that crosses from North to South Horizon and passes through the Zenith and Poles is the Local Meridian.

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Altitude and Azimuth• We can measure the

location of objects in the sky using pairs of angles. Here’s one way.

• Altitude is the angle above the horizon.

• Azimuth is the angle around the horizon clockwise from North. North is 12:00, East is 3:00

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Locations on Earth• We identify positions on

Earth using Latitude (Degrees North or South of the Equator) and Longitude (Degrees East or West of Greenwich, England).

• Greenwich is at the Prime Meridian (Longitude = 0°)

• PGCC about (39° North, 77° West)

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Navigation• Measuring the altitude of

the North Celestial Pole can give you your Latitude on Earth in the Northern Hemisphere.

• South of the Equator Polaris is never visible.

• The farther North you are the higher the North Celestial Pole appears in the sky.

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Locations in the Sky• We can measure locations

in the sky in a similar way.• Like Latitude, the

Declination is the angular distance from the Celestial Equator.

• Now we have to decide on a Celestial “Prime Meridian”.

• Astronomers choose one of the points where the Ecliptic crosses the Equator.

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Right Ascension & Declination• The Sun traveling along the

Ecliptic crosses the Equator at two points, the Vernal Equinox and the Autumnal Equinox.

• The Right Ascension is the angular distance from the Vernal Equinox

• Each position can be identified by its Right Ascension and Declination.