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Download The Passage of Time Moon, Moonth, Month Sidereal (Star) Time
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Lecture 4 1 2 The Passage of Time • Keeping track of time is one of the most fundamental tasks of astronomers. • Various astronomical cycles influence human activity: – day: most fundamentally the synodic day – month: the phases of the Moon – year: seasonal changes related to the Sun OWL #1 – Thursday Put your clicker numbers in OWL 3 4 Moon, Moonth, Month Sidereal (Star) Time • The 7 day week may have originated because it approximates the phases of the moon (Babylonians and Sumarians) • Not exact: Moon’s phase cycle is 29.5 days. • 12 lunar cycles is also close to a full year; 354 days instead of 365 (12 x 29.53059 = 354.36708) • History of calendars and astronomy shows social and cultural effects. • Most stars rise and set like the Sun does; we can measure time by a star’s position. • A difference arises because the Earth is orbiting the Sun…. 5 Why is the sidereal day shorter? Work with a partner to create a sketch which shows why the sidereal day is shorter than the solar day. You will need to show the motion of the Earth/where it is in its orbit for • (1) when a star crosses the meridian to the next crossing • (2) when the sun crosses the meridian to the next crossing 6 Sidereal vs. Synodic (Sun) Time Earth moves a little bit in its orbit each day, so the Sun appears to shift its position 1 7 8 Tilt affects where the Sun is on the Sky Star time vs. Sun time • The Sidereal day is ~4 minutes shorter than the Solar day. • There are 365.25 Solar days and 366.25 Sidereal days in a year. • The Sun shifts with respect to the stars by about 1 degree each day. Consequently, we see different constellations in winter than in summer! Cue Starry Night 9 Seasonal Change in Sun’s Altitude Where does the Sun rise different times of the year? 10 The Earth’s axis is tilted ~23.5 degrees relative to its orbit around the Sun. The axis remains pointed in the same direction (toward Polaris) as Earth orbits. As a result the Sun shifts from a declination of 23.5 deg N to 23.5 deg S over the year The tilt of the earth and time of year determine direction of sunrise & set. 11 PRS: Seasons 12 The Sun in January • Temperature changes are caused by changes in the total energy of sunlight we receive. • This can occur because of changes in: – (1) our distance from the Sun – (2) the directness of sunlight that we receive – (3) the number of hours of daylight • Which of these has a the most impact on Earth’s seasons? 2 13 The Sun in July 14 What part of the sky can you see? 15 The North Celestial Pole 16 Lost at Sea? Polaris is just about as high above the horizon as your lattitude. 3