Transcript “TIME”?
P/2010 A2 is unlike any object ever seen before. Close inspection shows a 140-meter nucleus offset from the tail center, very unusual structure near the nucleus, and no discernable gas in the tail. Knowing that the object orbits in the asteroid belt between Mars and Jupiter, a preliminary hypothesis that appears to explain all of the known clues is that P/2010 A2 is the debris left over from a recent collision between two small asteroids. Homework #2 is due Monday, Feb. 8, 2:30 pm If you could see stars during the day, the drawing below shows what the sky would look like at noon on a given day. The Sun is near the stars of the constellation Gemini. Near which constellation would you expect the Sun to be located at sunrise? A. Leo B. Cancer C. Gemini D. Taurus E. Pisces Sun Gemini Taurus Cancer Leo Pisces East South West Why do the stars appear to move? How fast do the stars move? Where would Gemini have been seen six hours earlier? Why does the Sun appear to move? How does the Sun move relative to the stars? Where would it have been located six hours earlier? Sun Gemini Taurus Cancer Leo Pisces East South West If you could see stars during the day, the drawing below shows what the sky would look like at noon on a given day. The Sun is near the stars of the constellation Gemini. Near which constellation would you expect the Sun to be located at sunrise? A. Leo B. Cancer C. Gemini D. Taurus E. Pisces Sun Gemini Taurus Cancer Leo Pisces East South West This homework question has been regraded as an “extra-credit” question D. Taurus E. Pisces Sun Gemini Taurus Cancer Leo Pisces East South West Tropic of Cancer – sun overhead at Summer Solstice Tropic of Capricorn – sun overhead at Winter Solstice Eclipses: understanding shadows Lunar phases arise from viewing the illuminated half of the Moon from different orientations. The Earth & Moon are also casting shadows into space. What are the consequences of these shadows? Eclipses occur when one object moves into the shadow of another object Lunar Eclipse: Moon in Earth's Shadow Parts of a shadow penumbra umbra Lunar Eclipse Three types: (1) Penumbral (2) Partial (3) Total Note that the edge of the Earth's shadow, as seen on the Moon during a partial eclipse, is curved. This was early evidence that the Earth is spherical. At what lunar phase(s) can lunar eclipses occur? Solar Eclipses Solar Eclipse: Earth in Moon's Shadow Three types of solar eclipses: (1) Total (2) Annular (3) Partial The widest the Moon's umbral shadow gets at the Earth is ~270 km, while the penumbral shadow ~7000 km For this reason, total and annular eclipses of the sun last only a few minutes at most and are seen over only a very small area Frequency of Eclipses If the Moon's orbital plane was the same as the Ecliptic plane, there would be two eclipses every month! Because of the inclination of the Moon’s orbit relative to the Ecliptic, the Sun, Earth and Moon are usually not truly lined up at New Moon and Full Moon Moon's orbital plane inclined by approximately 5 degrees relative to the Ecliptic Line of Nodes line connecting the two points where the Moon's orbit passes through the Ecliptic plane To have an eclipse, the Line of Nodes of the Moon's orbit must be lined up with Earth and Sun - “Eclipse Season”. Some type of lunar and solar eclipse will occur every eclipse season: it is usual to see two eclipses - one solar and one lunar - in an eclipse season, but roughly every seven years a third eclipse is possible If the Moon's orbit maintained the same orientation in space, eclipse season would occur every half year. However, the Moon's orbit precesses. This leads to eclipse seasons separated by approximately 173 days. Given the 29 ½ day lunar phase cycle (synodic period) and the ~ 173 day period of the eclipse season, there is an approximate concurrence of these cycles every ~18 yr 11 1/3 days This is called the Saros Cycle Total solar eclipses If you were on the Moon during a solar eclipse, what would you see? (a) A faint ruddy reddish Earth (b) Nothing different (c) A small circular shadow moving across the Earth's surface (d) A bright ring surrounding a darkened Earth. If you were on the Moon during a solar eclipse, what would you see? (a) A faint ruddy reddish Earth (b) Nothing different (c) A small circular shadow moving across the Earth's surface (d) A bright ring surrounding a darkened Earth. Intermission with entertainment! The very first ever Blonde GUY joke..... And well worth the wait! An Irishman, a Mexican and a Blonde Guy were doing construction work on scaffolding on the 20th floor of a building. They were eating lunch and the Irishman said, "Corned beef and cabbage! If I get corned beef and cabbage one more time for lunch, I'm going to jump off this building." The Mexican opened his lunch box and exclaimed, "Burritos again! If I get Burritos one more time I'm going to jump off, too." The blond opened his lunch and said, " Bologna again! If I get a bologna sandwich one more time, I'm jumping too." The next day, the Irishman opened his lunch box, saw corned beef and cabbage, and jumped to his death. The Mexican opened his lunch, saw a Burrito, and jumped, too. The blonde guy opened his lunch, saw the bologna and jumped to his death as well. At the funerals, the Irishman's wife was weeping. She said, "If I'd known how really tired he was of corned beef and cabbage, I never would have given it to him again!" The Mexican's wife also wept and said, "I could have given him tacos or enchiladas ! I didn't realize he hated Burritos so much." Everyone turned and stared at the blonde's wife. The blonde's wife said … "Don't look at me. He makes his own lunch" TIME What exactly do we mean by the term “TIME”? What approach would you take to keeping track of time on short time scales? Long time scales? Are there advantages or disadvantages to these approaches? Why is this important (or is it)? Keeping track of time Critical to societies in which the availability of food and weather have seasonable variability and to societies with rapid communications and travel Over 5,000 years ago civilizations started the quest for accurate ways to tell the time of day as Earth rotates on its axis. They used observations of the Sun and other heavenly bodies to indicate times like noon. The challenge was how to divide the day into regular units that could be synchronized, even with cloudy days. Time keeping is based on cycles in the sky Day: successive meridian crossings of sun (solar day) or star (sidereal day) The Sun rises about 4 minutes later each day as compared to the stars Time keeping is based on cycles in the sky Day: successive meridian crossings of sun (solar day) or star (sidereal day) Year: rising/setting points of sun on horizon Month: cycle of lunar phases 24 HOUR DIVISION OF THE DAY Around 1500 B.C., Egyptians developed a sundial, onto which they divided the daylight hours into 10 equal parts. 24 HOUR DIVISION OF THE DAY Around 1500 B.C., Egyptians developed a sundial, onto which they divided the daylight hours into 10 equal parts. They designated two additional parts (“hours”) to signify twilight time (morning & evening) They divided the night time into twelve portions based upon crossing of the meridian by evenly spaced “clock stars” Ever since then, we have divided the day into twentyfour portions (hours) GOT HERE Why do clocks run in the “clockwise” direction? Apparent Solar Time vs Mean Solar Time Apparent Solar Time: Based on the location of the sun in the sky relative to the local meridian Because of the Earth’s variable orbital speed (due to its noncircular orbit) and to the inclination of Ecliptic to Equatorial plane, the rate at which the sun appears to move is not uniform. This leads to variable length days! Not very useful to have hours and days that are not uniform in length! Solution: Create fictitious sun which moves at a uniform rate equal to the mean motion of the sun. Mean Solar Time Location of “average” sun relative to the local meridian. This average sun moves at a constant speed relative to the celestial equator – equal length days. Mean solar time can run from 17 minutes earlier than apparent solar time to 15 minutes later. Relationship between two is given by the “analemma” Both apparent and mean solar time are defined locally. Need more uniform time keeping scheme. Standard Time: Time zones within which the time is approximately the same as the mean solar time at the center of the zone.