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1. Aries = (March 21 - April 20 )2. Taurus = (April 21 - May 21)3. Gemini = (May 22 - June 21 )4. Cancer = (June 22 - July 22 )5. Leo = (July 23 -August 21 )6. Vigro = (August 22 - September 23 )7. Libra = (September 24 - October 23)8. Scorpio = (October 24 - November 22 )9. Sagittarius = (November 23 - December 22)10. Capricorn = ( December 23 - January 20 )11. Aquarius = (January 21 - February 19 )12. Pisces = ((February 20- March 20

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Is not a property of stars themselves.

It is an effect of our Earths atmosphere.

Stars twinkle.

Planets do not usually seem to twinkle.

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We need to know an objects direction to

observe it.

Celestial sphere: large imaginary sphere, on

which all astronomical objects hung on itsinside surface.

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Longitudes: half circles run from north

pole to south pole.

Longitude 0 passes through Greenwich.

Dhahran Longitude +49

54 33 (East)

Latitudes: parallel circles that run

around Earth, all parallel to the equator.

Latitude 0 is the equator Latitude 90 the poles

Dhahran Latitude +26 24 32 (North)

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Right ascention = Longitude (East West)

Declination = Latitude (North South)

The North Celestial Pole is the pointon the celestial sphere directly above

the Earth's North Pole.

The South Celestial Pole is directlyabove the Earth's South Pole. The star Polaris, in the constellation

Ursa Minor, is located very close to

the North Celestial Pole.

The celestial equator is directlyabove the Earth's equator.

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Declination corresponds to latitude

and is measured in the same way, butrelative to the celestial equator (0dec).

The north celestial pole is at 90 northdeclination (+90 dec). The southcelestial pole is at 90 south declination

(-90 dec).

Circles of constant declination are allparallel to the celestial equator.

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For any position on the surface of the

Earth, the point on the celestial

sphere that is directly overhead is

called the zenith.

Since the Earth and the celestial

sphere are concentric, simple

geometry shows that the zenith willalways have a declination equal tothe latitude of the observer (such asfor Atlanta in the picture).

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A star's position along a circle of constant declinationis described by a second number called rightascension.

Right ascension corresponds to longitude, but

different units are used.Instead of 360, a circle is broken into 24 hours ofright ascension.

So, 360 = 24 h R.A., 15 = 1 h R.A., and 1 = 4 min

R.A.Note that hours of right ascension is a unit ofangle,not time, although there is an obvious connectiondue to the daily rotation of the celestial sphere.

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Right ascension is measured fromthe celestial meridian, chosen to be 0 h R.A.

(which is also the same as 24 h R.A.)

The celestial meridian is a semicircleconnecting the celestial poles and passingthrough a particular point on the celestial

equator called the vernal equinox. Right ascension increases from west to east

(note that we are looking at the exteriorofthe celestial sphere in the above picture).

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Although the stars are fixed relative to each

other, the Sun moves relative to the stars.

Once a year, the Sun traces out a circle on

the celestial sphere called the ecliptic.

The ecliptic is tilted at an angle of 23.5with respect to the celestial equator.

(The Moon and planets also move near the

ecliptic.)

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The Sun crosses the celestial equator at exactlytwo points, called equinoxes, from the Latinfor "equal nights.

The equinox where the Sun ascends from thesouthern to the northern hemisphere is calledthe spring or vernal equinox because the Sunis there on March 21.

The vernal equinox is chosen to be 0 h R.A.

The Sun again crosses the celestial equatorhalfway around, at 12 h R.A.

This position is called the autumnalequinox because the Sun is there on September

23.

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The positions where the Sun reaches its highestand lowest points are called solstices, from theLatin for "the Sun stops" as it changes direction.

The Sun is highest in the sky (in the northernhemisphere) when it is at 6 h R.A.This position is called the summersolstice because the Sun is there on June 21.The Sun then has a declination of +23.5.

The Sun is lowest in the sky (in the northernhemisphere) when it is at 18 h R.A.This position is called the wintersolstice because the Sun is there on December21.

The Sun then has a declination of -23.5.

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Event Date RA Dec

VE 21 Mar. 0 h0

Equator

SS 22 Jun. 6 h+23.5

Tropic of cancer

AE 23 Sep. 12 h 0Equator

WS 22 Dec. 18 h-23.5

Tropic of Capricorn

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The altitude of an object is the anglebetween it and the horizon.

The horizon has an altitude of 0 and the

zenith has an altitude of 90.

The azimuth of an object is the anglebetween it and north, measured clockwise

along the horizon.

North has an azimuth of 0, east has anazimuth of 90, south has an azimuth of180, and west has an azimuth of 270.

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J2000 RA: 6h45m08.90s DE:-1642'58.0"

Date RA: 6h45m38.31s DE:-1643'41.2

Dhahran 2010-12-20 2h59m

Sideral Time : 9h13m Azimuth :+22347'

Altitude :+3359

Rise : 19h04m Azimuth:+10827'

Culmination : 0h32m

Set : 6h00m Azimuth:+25133

========================================

J2000 RA: 6h45m08.90s DE:-1642'58.0"

Date RA: 6h45m38.38s DE:-1643'41.4

Dhahran 2010-12-30 2h59m

Sideral Time : 9h52m

Hour Angle : 3h07m Azimuth :+23143'

Altitude :+2727

Rise : 18h24m Azimuth:+10827'

Culmination : 23h53m

Set : 5h21m Azimuth:+25133

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J2000 RA: 6h23m57.10s DE:-5241'45.0"

Date RA: 6h24m11.70s DE:-5242'08.0"

Dhahran 2010-12-20 2h59m

Sideral Time : 9h13mHour Angle : 2h49m

Azimuth :+20402'

Altitude :+0300' Rise : 20h48m Azimuth:+15203'

Culmination : 0h11m

Set : 3h33m Azimuth:+20757

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Period of 26000 years.

Small correction of celestial

positions is needed.

Small change (1/60 peryear).

Latest is epoch 2000

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Sep. 22 Autumnal Equinox

Sun has Dec. of 0 and RA of 12 h

Sirius has Dec. of -16 43 and RA of6h 45m

A) what are the RA and Dec of the Sun onMarch 21?

B) what are the RA and Dec of the Sun on

June 22?

C) What are the RA and Dec of Sirius on June

22?

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Bernards Star has R.A. 17h 57.9m and dec.

+4 41

It crosses the meridian facing south at

sidereal time 17h 57.9m

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At what solar time does Bernards star cross

due south of you on September 22nd?

It crosses the meridian facing south at

sidereal time 17h 57.9mOn Sep. 22nd it corresponds to solar time 17h

58m

On Sep. 26th it corresponds to solar time 17h

42m (16 min earlier)

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The sun Today has RA of approximately 18 h

At what sidereal time does the sun cross the

Meridian due south, today.

Answer: at noon 11:40 am solar time = 18 hsidereal time.

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Sirius has RA 6h45m38.31s

It will cross the Meridian due south at

6h45m38.31s (sidereal time) wich is

equivalent to 12:08 a.m.

In June: it will cross the Meridian at

6h45m38.31s (sidereal time) which is

equivalent to 12:23 p.m.