AST 115 1st Edition Lecture 3Outline of Last Lecture I. Illusions in the night sky continuedII. Constellations of the ZodiacIII. Coordinate/Angle systema. Equatorial CoordinatesIV. Daily Paths of objects in the skyOutline of Current Lecture I. Measuring latitudeII. Circumpolar starsIII. Seasonsa. Earth’s tiltIV. Phases of the Moona. Causesb. Phases on other planetsV. View of the Earth from the surface of the MoonVI. Synodic vs. SiderealVII. Introduction to measuring timeCurrent Lecture How to measure your latitudeo Measure the altitude of Polaris above your northern horizon Your latitude = altitude of NCP Circumpolar Starso Circumpolar stars are stars that go around the poleso Never rise and never seto At the North and South Poles, the entire sky (1/2 of the Celestial Sphere) is circumpolar.o At the equator, none of the sky is circumpolar.o At Springfield, Missouri’s latitude, a small section of the northern sky is circumpolar. Seasonso What causes the changing of the seasons on Earth? The changing of the seasons is NOT due to the distance of the Earth from the Sun.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. The changing of the seasons is NOT due to any change in the Sun’s outputof heat. The changing of the seasons is due to the fact that Earth’s rotational axis is tilted about 23.5˚ o Because of this tilt: The duration of daylight varies annually - At our latitude, the Sun is above the horizon about 15 hours during the summer months, but only 9 hours during the winter months. The angle of the sunlight that strikes the Earth’s surface varies- At our latitude, the Sun’s altitude at noon is about 77˚ in the summer, but only about 30˚ in the winter.o The current season in the southern hemisphere is always the opposite of ours. Phases of the Moono When the Sun shines on any sphere, there is always a sunlit hemisphere and an unlit hemisphere.o The phases of the Moon are caused as the Moon orbits the Earth and we see a changing portion of the sunlit hemisphere. The phases of the Moon have nothing to do with the Earth’s shadow.- The only time the Earth’s shadow plays a role in the phases of the Moon is during a lunar eclipse.o The other planets go through phases tooo The phase shapes for Mercury and Venus are quite obvious, but the outerplanets have phases that are slightly gibbous and full. The view of the Earth from the surface of the Moono Size Earth would look 4 times as big as the Moon looks to us on Eartho Color Earth would look quite colorful; bright white clouds and polar ice caps, yellows, blues, and greens.o Brightness Earth is highly reflective, so it would look much brighter from the surface of the Moon.o Phases The phase would be opposite of the Moon seen from Earth.o Position in the lunar sky Earth would appear to hang low in the sky. Synodic vs. Siderealo Synodic period = with respect to the Suno Sidereal period = with respect to the starso Concerning the Moon’s orbit of the Earth: Synodic period = about 29 and ½ days (same as the period of the cycle of the phases) Sidereal period = about 27 and 1/3 dayso The Moon’s rotation (spin) is synchronized to its revolution (orbit). Time measuredo Time systems Local Apparent Solar Time (Sundial)- L.A.S.T. = the hour angle of the Sun + 12 hours- Time system that speeds up and slows down (it’s not very consistent). Local Mean Solar Time- L.M.S.T. = the hour angle of the “mean” of the Sun + 12 hours- This time system runs at a constant rate- Problem = it’s local, which means it’s different everywhere you go Standard Time- There are 24 standard time zones - Central Standard Time = CSTo CST = LMST for 90˚ W longitude- Daylight Savings Time o DST = Standard Time + 1
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