Astro Ch2.pdf Astr 1010
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This 5 page Class Notes was uploaded by Alikhan Ladhani on Tuesday January 26, 2016. The Class Notes belongs to Astr 1010 at Georgia State University taught by in Spring 2016. Since its upload, it has received 56 views. For similar materials see Astronomy in Science at Georgia State University.
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Date Created: 01/26/16
2.1 PATTERNS IN THE NIGHT SKY Constellations • Constellations- region of the sky with well defined borders • Names and borders of the 88 official constellations were made in the 1928 by the members of the international astronomical union (IAU) o Most lived in the Europe or US o Constellations in the northern hemisphere can be traced back to civilization of ancient middle east while the southern hemisphere are named by the European explores in the 17 thcentury. The Celestial Sphere • Constellation appear to lie close to one another but are far apart because they may lie at very different distances from earth o Due to the lack of depth perception when looking in to space • Useful to map the stars in space • North celestial pole- directly over earths north pole • South celestial pole- over south pole • Celestial equator- projection of earths equator into space • Ecliptic- path the sun follows as it appears to circle around the celestial sphere once a year. Crosses celestial equator at 23.5° angle The Local Sky • Local sky- the sky as seen form wherever you happen to be standing • Horizon- boundary between earth and sky • Zenith- point directly over head • Meridian- imaginary half circle stretching from horizon due south through zenith to horizon due north • To pinpoint a object in the sky you state its direction along the horizon (sometimes stated as azimuth degrees clockwise from due north) and altitude Angular Sizes and Distances • Angular size- the angle it appears to span in your field of view o Does not tell us the true size of an object because it also depends on distance • Size of sun and moon ½° o The sun is 400x bigger than the moon but it is also 400x farther away which gives the sun the same size as the moon • Angular distance-the angle that appears to separate objects • Precise astronomical measurements subdivide each degree into 60 arcminutes and subdivide that into 60 arcseconds o 1°=60’ (arcseconds) o 1’=60’’ (arcminutes) o Ex. 35° 27’ 15’’ read as 35 degrees, 27 arcminutes, 15 arcseconds • Small angle formula (valid only when the angular size is small) ▯▯▯▯▯▯▯ ▯▯▯▯▯▯▯▯▯▯▯▯ ▯▯▯▯ o ▯▯▯° = ▯▯ ▯ ▯▯▯▯▯▯▯▯ ▯▯▯° o ???????????????????????????? ???????????????? = ????ℎ???????????????????????? ???????????????? ????▯▯ ▯ ▯▯▯▯▯▯▯▯ • Stars near the north celestial pole are circumpolar. They remain perpetually above the horizon circling (counterclockwise) around the north celestial pole each day • Stars near the south celestial pole never rise above the horizon at all • All other stars have daily circles that are partly above the horizon and partly below it meaning they appear to rise in the east and set in the west Variation with Latitude • Latitude- measures north- south position o 0° at equator increasing to 90°N at the north pole and 90°S at the south pole o Latitude affects the constellation we see because it affects the locations of the horizon and zenith relative to the celestial sphere • Longitude measures east- west position o 0° along the prime meridian which passes through Greenwich England 2.2 THE REASON FOR SEASONS • The reason for season is because the tilt of earths axis causes sunlight to fall differently on earth at different times of the year • Northern hemisphere tipped towards the sun in June and away in December Solstices and Equinoxes • 4 special moments in the year o Spring (march) equinox- around march 21 northern hemisphere goes from being tipped slightly away form the sun to being tipped slightly toward the sun o Summer (June) solstice- occurs around June 21, northern hemisphere is tipped toward the sun (receives most direct sunlight) o Fall (September) equinox- around September 22, northern hemisphere first starts to be tipped away from the sun o Winter (December) solstice- around December 21, (northern hemisphere receives least direct sunlight) • Precession- gradual wobble that alters the orientation of earths axis in space • Precession does not change the amount of the axis tilt (which stays close to 23 ½°) does not affect pattern of the season • Law of conservation of angular momentum objects tend to keep spinning around the same rotation axis 2.3 THE MOON, OUR CONSTANT COMPANION Lunar phases- moon appearance in our sky changes as its position relative to the sun changes which happens every 29 ½ days (origin of the word months) The Moons Synchronous Rotation • Synchronous rotation- moon rotates on its axis in the same amount of time it takes to orbit earth o The same side of the moon is always facing earth The View From The Moon • The dark portion of the lunar face is not totally dark • Sunlight reflected by earth faintly illuminates the dark portion of the moon o Ashen light, earthshine enables us to see the outline of the full moon even when the moon is not full What Cause Eclipses • Eclipses- when the moon and earth cast shadows in sunlight o Happens when the sun earth and Moon fall into a straight line o Lunar eclipse occurs when earth lies directly between the sun and moon so earths shadow falls on the moon o Solar eclipse occurs when the moon lies directly between the sun and the earth so the moon shadow falls on earth Condition For Eclipses • Moons orbit is slightly inclined about 5° to the ecliptic plane (earths orbit around the sun) o Due to this the moon spend most of the time above or below the ecliptic plane. Crosses through twice once going up and going down o Nodes- two points in each orbit at which the moon crosses the ecliptic plane • Condition for an eclipse o Phases of the moon must be full (for lunar eclipse) or new (for solar eclipse) o New or full moon must occur during one of the periods when the nodes of the moon orbit are aligned with the sun and earth Inner and Outer Shadows • Umbra- sunlight is completely clocked (dark section in picture) • Penumbra- sunlight is only partially blocked (light gray in picture) Lunar Eclipses • Lunar eclipse begins when the moon orbit first carries it into earths penumbra and then there is one of three lunar eclipses o Total lunar eclipse- sun earth and moon nearly perfect aligned the moon passes through earths umbra (moon becomes dark and strangely red) o Partial lunar eclipse- alignment is somewhat less perfect part of the full moon passes through the umbra (with the rest in penumbra) o Penumbral lunar eclipse- moon passes through only earths penumbra (most common) • Totality- when the moon is entirely engulfed in the umbra Solar Eclipses • When the earth is close to the moon, the moons umbra can cover 270 kilometers in diameter of earths surface total solar eclipse • Annular eclipse- when the moon is farther away from earth, the umbra may not reach earths surface (ring of sunlight surrounding the moon) • Partial solar eclipse- only part of the sun is blocked 2.4 THE ANCIENT MYSTERY OF THE PLANETS • Planets usually move eastward but on occasion they reverse course moving westward through the zodiac lasting few weeks to a few months – apparent retrograde (means backward) motion • Planets do not move backward actually, it only appears that it does
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