Locating Celestial Objects

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LocatingCelestial Objects


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In this lesson, you will learn...

what the celestial sphere and celestial equator

are, and terminology related to them

how to describe the location of objects in the

night sky using both the Declination - Right

Ascension system and the Altitude - Azimuth

system

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Looking Up...

Early astronomers believed that all of the stars we

see in the night sky were fixed on a celestial sphere

surrounding the Earth.

Although we now know that this is not true, it iseasier to use this idea to locate stars in the sky.

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The Celestial Sphere

The celestial sphere is a dome that surrounds

the Earth.

It is divided in much the same way as we

divide Earth with the North and South

celestial poles as an extension of the Earths

axis.

The celestial equator is a projection of Earths

equator onto the sphere.

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The

Celestial Sphere

showing the

celestial North

and South poles

and the

celestial equator.

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As the Earth rotates inside the celestial sphere

(or, as early astronomers believed, the sphere

rotates around us) the stars appear to rise in

the east and then set in the west.

This of course includes our own star, the Sun.

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Locating Stars

Astronomers use a coordinate system similar

to the latitude-longitudesystem used on Earth

to locate stars in the sky.

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There are two different

coordinate systems:

But first, a little terminology

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The eclipticis the apparent path of the Sun in one year. It is also

along the plane in which the Earth revolves around the Sun.

The summer solstice is the day of the year that the northern

hemisphere gets the most sun.

The winter solstice is when we get the least sunlight (and the

southern hemisphere gets the most).

The vernal equinox isthe day in the spring

when we get exactly 12

hours of day and 12

hours of night.

The autumnal equinox

is the same thing, but

occurs in the fall

(autumn).

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System 1:

Declination - Right Ascension

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Declination

Declination is analogous to latitude and is

measured in degrees from the celestial

equator.

Measurements of declination are either

positive (north) or negative (south).

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Right Ascension

Right ascension is analogous to longitude and

is measured in hours, minutes, and seconds,

from the point of vernal equinox (where the

ecliptic and celestial equator meet).

Since a circle is 360o(and it has been broken

into 24 parts) each hour is 15o.

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Breaking that hour (15o) into 60 pieces gives

us minutes, and breaking minutes into 60

pieces given us seconds.

For example:

If the star was

located at an angle

of 50o, this would

equate to 3 hours

or 3h 20 m.

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Example:

The star in this

picture has...

Declination: 45o

Right Ascention: 3h

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System 2: Altitude-Azimuth

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Altitude

Altitudeis the angular distance (measured

from 0oto 90o) of a celestial object above the

observers horizon.

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Azimuth

Azimuthis the compass angle from due N to

the location of the celestial object.

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Example:

The star in this picture appears to have an altitude of

approximately 40o(which means it's zenith angle is

50o) and an azimuth of about 200o(just over halfway

around the circle, starting from North and goingclockwise).

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Angular Sizes We can measure angles in the sky

The angular sizeof an object like the Sun or

the Moon is the angle it appears to span in

your field of view.

The angular distance between a pair of

objects is the angle that appears to separate

them..


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Angular size of

moon ~ o

Angular distance between the

pointer stars ~ 5o

To

Polaris

Locating Celestial Objects

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