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Lecture 5

Lecture 5.docx

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Bozidar Mitrovic

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Monday September 16, 2012 Astronomy Lecture 5: Night Sky: On a clear night one observes a) Stars (deneb, vega, altair…) b) Constellations: groups of stars, which lie in approximately the same direction (the only thing which they have in common!) Examples: Cassiopeia (W shaped) Cepheus (square structure with a triangle) Ursa major (its brightest stars form an asterism – big dipper) Ursa minor (its brightest stars form – little dipper) There are 88 constellations, 44 of which were known in the ancient times (we take their names from Greek version translated into Latin) What one observes in the night sky depends on the (geographical) latitude (St.Catharines latitude is 43.167 degrees North) Point directly over top of an observer is called the zenith (altitude= 90 degrees) Horizon (altitude = 0 degrees) Meridian altitude = 60 degrees direction SE Altitude of an object (or point) in the sky is its angle above the horizon Because the earth is rotating (spinning) about N-S axis from west to east and because it is also revolving around the sun there are two types of star and sun motions 1) Daily – caused by rotation of the earth ( stars move by 360 degrees/24 h = 15 degrees/h) 2) Annual – caused by revolution of the earth around the sun (Stars and constellations move by 360 degress/365.25days = 1 degree/day) Wednesday September 19 2012th To describe the daily (and annual) motions of stars we use the concept of Celestial Sphere – an imaginary sphere centered on earth with a radius much larger than the radius of the earth North Celestial pole – a point on celestial sphere directly above the north pole Stars are revolving around the NCP At present (over the last few hundred years and over the next few hundred years) the star very close to NCP is polaris (the north star) . In the northern hemisphere the stars appear to move around NCP in the counterclockwise direction. In the southern hemisphere the stars apper to move around the SCP (south celestial pole) in the clockwise direction (SCP is a point on celestial sphere directly above the earth’s south pole) Since the earths rotational axis (N-S axis) is perpendicular to equatorial plane it appears that the stars are moving parallel to the celestial equator Celestial equator (directly above the earth’s equator) is the same line for ALL observers On the other hand, celestial horizon will depend on the location of the observer Only when you are at the north pole or south pole will the Stars close enough to one of the celestial poles never rise or set. They are called circumpolar stars Observer at the north (or south) pole: all stars are circumpolar (no star rises or sets) Observer on the equator: NCP and SCP are on his horizon and since the stars move about NCP-SCP axis all stars rise and set – there are no circumpolar stars At intermediate latitudes (between 0 degrees and 90 degrees): some stars are circumpolar (if they are close enough to the celestial pole) and the others rise and set Friday September 21, 2012 Annual motions of stars: Annual motions of stars resulting from the earth’s revolution around the sun (the stars move by 360°/365.25 days = 1°/day) The results of this motion are: 1) Old constellations set in the west and the new ones appear in the east 2) Each rising star rises about 4 minutes (3 minutes and 56 seconds) earlier every night; the stars also set by about 4 minutes earlier each night. As a result, relative to the stars the sun is moving eastward The projection of the earth’s orbit on celestial sphere is called ec
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