1 Lecture 2 Motions in the Sky September 10, 2018
2 What is your year in school? A. New freshman B. Returning freshman C. Sophomore D. Junior E. Senior F. I ve been here, like, forever
3 What is your major? A. Arts or Communication B. Natural Resources C. Professional Studies D. One of the Sciences E. One of the Humanities (languages, philosophy, political science, etc.) F. Undeclared
4 Why are you taking this class? A. Astronomy seems interesting to me. B. Only because it satisfies my GEP requirements. C. It was the only class that would fit in my schedule. D. A friend recommended it to me. E. Some other reason.
5 Units of Measure We ll use mainly the metric system, based on the meter, kilogram, and second We ll often use scientific notation: 1 light-year = 9,461,000,000,000,000 m = 15 9.461 10 m You should know the metric prefixes and scientific notation
6 Angular Size Astronomers measure angles in degrees and arc minutes (60 per degree) and arc seconds (60 per arc minute) The size of an object is related to its distance from Earth and its observed angular diameter physical diameter= distance angular diameter 180 D = r 57.3 r D
7 Estimating Angles
8 Jupiter has a physical diameter of 143,000 km and is 635,000,000 km from Earth. What is its angular size? A. 46.5 arcsec B. 129 arcsec C. 635 arcsec D. 1910 arcsec
9 Jupiter has a physical diameter of 143,000 km and is 635,000,000 km from Earth. What is its angular size? A. 46.5 arcsec B. 129 arcsec C. 635 arcsec D. 1910 arcsec r 57.3D D = 57.3D = r = 57.3 r ( 5 ) 57.3 1.43 10 km = = 0.0129 8 6.35 10 km 3600 arcsec = 0.0129 = 46.5 arcsec 1
10 Yearly Motions The Earth travels around the Sun once every year. The Earth is tilted by 23.5 degrees with respect to the orbital plane. E E E E
11 The Sun lines up with different constellations during the year. Presently These stars are overhead during the day. Sun is too bright to see them. These stars are visible at night. 6 months later These stars are visible at night. These stars are overhead during the day. Sun is too bright to see them
12 What is a Day? Solar Day = amount of time for Earth to rotate once on its axis with respect to the Sun. Sidereal day = amount of time for Earth to rotate once with respect to the stars. Shorter than solar day by 4 minutes. Since solar and sidereal days are NOT the same, a star will typically rise 4 minutes earlier each day.
13 Ecliptic Path of the sun through the celestial sphere. Not aligned with Celestial Equator due to tilt of the Earth
14 Zodiac The 13 constellations through which the sun passes during the course of a year. Interactive Figure (might not work in every browser)
15 The Real Signs of the Zodiac Constellation Traditional Actual (2000) Days Capricornus Dec. 22-Jan. 21 Jan. 21-Feb. 16 26 Aquarius Jan. 22-Feb. 21 Feb. 16-Mar. 11 24 Pisces Feb. 22-Mar. 21 Mar. 11-Apr. 18 38 Aries Mar. 22-Apr. 21 Apr. 18-May 13 25 Taurus Apr. 22-May 21 May 13-June 22 40 Gemini May 22-June 21 June 22-July 21 29 Cancer June 22-July 21 July 21-Aug. 10 20 Leo July 22-Aug. 21 Aug. 10-Sept. 16 37 Virgo Aug. 22-Sept. 21 Sept. 16-Oct. 31 45 Libra Sept. 22-Oct. 21 Oct. 31-Nov. 23 23 Scorpius Oct. 22-Nov. 21 Nov. 23-Nov. 29 6 Ophiuchus Nov. 29-Dec. 18 19 Sagittarius Nov. 22-Dec. 21 Dec. 18-Jan. 21 34
If we could observe background stars in daylight, how would the Sun appear to move against this background because of the Earth s orbit around the Sun? A. 1 per day, from west to east B. 1 per day, from east to west C. 15 per hour, from west to east D. 15 per hour, from east to west 16
17 Planetary Motion The planets all move in the same plane about the Sun so they all appear to move along the zodiac (near the ecliptic).
18 Summer Solstice Longest period of daylight in Northern Hemisphere North Celestial Pole Summer Solstice: Sun reaches highest point in sky for N. Hemisphere. (June 21) Celestial Equator Ecliptic South Celestial Pole E
19 Autumnal Equinox Equal periods of day and night for entire Earth North Celestial Pole Autumnal Equinox: Sun crosses Celestial Equator. (September 21) Celestial Equator South Celestial Pole Ecliptic E
20 Winter Solstice North Celestial Pole Longest period of darkness in Northern Hemisphere Winter Solstice: Sun at lowest point in sky for N. Hemisphere. (December 21) Celestial Equator Ecliptic South Celestial Pole E
21 Vernal Equinox Equal periods of day and night for entire Earth North Celestial Pole Vernal Equinox: Sun crosses Celestial Equator. (March 21) Celestial Equator South Celestial Pole Ecliptic E
The ecliptic crosses the celestial equator at A. Two points, known as solstices B. Two points, known as equinoxes C. One point, known as the vernal equinox D. One point, known as the meridian 22
23 The Seasons The Seasons are due to the tilt of the Earth with respect to the orbital plane. Not distance from sun Flash Applet
24 Why are the seasons warm and cold? During the Summer the sun is higher in the sky so the light is more direct.
25 Why are the seasons warm and cold? Sun is above the horizon longer in the summer.
26 During summer in the northern hemisphere, why is it still cold at the North Pole, even though they get 24 hours of daylight? A. The Earth is further from the Sun in the summer. B. The North Pole is further from the Sun than Stevens Point C. The light is still very indirect even in the summer. D. None of the above.