Lecture 2: Motions of the Earth and Moon. Astronomy 111 Wednesday August 30, 2017

Transcription

1 Lecture 2: Motions of the Earth and Moon Astronomy 111 Wednesday August 30, 2017

2 Reminders Online homework #1 due Monday at 3pm Labs start next week

3 Motions of the Earth ASTR111 Lecture 2

4 Observation: Stars, Sun, Moon and planets move in counterclockwise circles around the north (and south) celestial pole. Objects generally move east to west when above the horizon ( rising in east, setting in west). What causes these circular motions?

5 Motions of the Earth The Earth rotates on its axis This motion causes: Sunrise and sunset Motion of stars

6 Stars rise and set ASTR111 Lecture 2

7 Observation: One star in the sky never moves the North Star or Polaris Why is Polaris fixed in position?

8 Earth s axis is fixed ASTR111 Lecture 2

9 Question: Where was this picture taken?

10 Celestial navigation made simple At Earth s North Pole: Polaris is directly overhead At Earth s Equator: Polaris is due north, on the horizon In Earth s Northern hemisphere: Polaris is due north: its height above the horizon (in degrees) is equal to your latitude (in degrees)

11 Earth s rotation causes apparent motion Remember: the stars and the Sun DO NOT orbit the Earth once per day Rather, the Earth rotates once per day Produces apparent motions of stars, the Sun, planets

12 The celestial sphere ASTR111 Lecture 2

13 The celestial sphere Thinking about the stars we see in the night sky

14 The celestial sphere: a large imaginary sphere around Earth ASTR111 Lecture 2

15 Special locations on the celestial sphere North Celestial Pole = point directly above Earth s North Pole (near the star Polaris) South Celestial Pole = point directly above Earth s South Pole (no nearby bright star) Celestial Equator = circle directly above Earth s Equator

16 Locations on celestial sphere ASTR111 Lecture 2

17 Distances between points on the celestial sphere are measured in degrees, arcminutes, and arcseconds 360 degrees in a circle 60 arcminutes in a degree 60 arcseconds in an arcminute ½ degree = angular size of Sun & Moon

18 The night sky ASTR111 Lecture 2

19 The sky is divided into 88 constellations Mesopotamia, circa 3000 BC: oldest know constellations Ptolemy, 2nd century AD: 48 constellations in Northern sky 16th to 18th century AD: unmapped (mostly Southern) regions of sky filled in Telescopium, Microscopium, Reticulum!

20 Hipparchus Hipparchus (ca. 140 BC) was perhaps the greatest astronomer of ancient times. He produced a catalogue of 1000 stars and classified them according to their apparent brightness. Modern usage still retains names of stars and rankings by brightness within a constellation, although Arabic names are also used for the brightest stars

21 Constellations are arbitrary Other cultures, other constellations: Emu Southern Cross Stars in a constellation usually are not at the same distance from us. At a different place in our Galaxy, we would see different star patterns.

22 Observation: Sun moves west to east relative to stars (about 1 degree per day). What causes this annual motion?

23 Two possible explanations of 1 year cycle: HYPOTHESIS #1 Sun revolves around Earth at a slightly slower rate than the celestial sphere. WRONG! HYPOTHESIS #2 Earth revolves around the Sun, once per year. RIGHT!

24 A model of the celestial sphere, showing the constellation borders, the celestial equator, and the apparent path of the Sun against the background of constellations.

25 The Sun appears to move west to east relative to stars (1 year cycle) Today the Sun is in a particular constellation, next month in a different one, etc. Sun s path on the celestial sphere = ecliptic Constellations through which the ecliptic runs = zodiac The ecliptic is NOT the same as the celestial equator!

26 The Zodiac The Sun is in the direction of the constellation Virgo in September, as viewed from the Earth. That is why people born in September are said to be born under the astrological sign of Virgo.

27 Observation: The weather in the summer is hotter than in the winter (especially in Texas!) What is the cause of the seasons on Earth?

28 Earth s tilt causes seasons Remember that the Earth's axis of rotation is tilted 23.5 degrees to the plane of its orbit about the Sun.

29 Earth s tilt causes seasons ASTR111 Lecture 2

30 Earth s tilt causes seasons When the Northern hemisphere is tilted towards the Sun we have summer in the Northern hemisphere and winter in the Southern hemisphere. When the Northern hemisphere is tilted away from the Sun we have winter in in the Northern hemisphere and summer in the Southern hemisphere. This is the cause of the seasons.

31 Earth s tilt causes seasons ASTR111 Lecture 2

32 Earth s tilt causes seasons The reason it is warmer in summer than in winter is that the sunlight is more concentrated on the ground when the Sun is higher in the sky.

33 Seasons The first day of spring occurs on about March 20th or March 21st. (In the Northern hemisphere, this is called the vernal equinox.) The first day of autumn occurs about September 22nd. (This would be the first day of spring in The Southern hemisphere, and the autumnal equinox in the Northern hemisphere.) Equinoxes occur when the declination of the Sun is zero degrees (i.e. located on the celestial equator).

34 Observation : The location of Polaris moves (over thousands of years) the North Star was not always the North Star! What causes this motion?

35 Precession Spinning top takes <1 sec to precess... Earth takes 26,000 years!

36 Precession Hipparchus compared the coordinates of some stars with records made by the Babylonians and discovered that the ecliptic longitudes of the stars were increasing with time, about 1 degree per century. (The modern value is about 1 degree in 72 years.) This is called precession (not to be confused with the word precision ). The Earth turning on its axis is like a spinning top.

37 Precession While Polaris is close to the north celestial pole now, that was not always the case. Due to the 26,000 year period of precession, many stars take their turns being the pole star.

38 Observation: There is a large bright object in the nighttime (and often the daytime) sky How does the Moon move?

39 Motions of the Moon ASTR111 Lecture 2

40 The Moon appears to move west to east relative to stars (1 month cycle) Today the Moon is in some constellation In two weeks: a different constellation In four weeks: first one, again. Observation: Moon moves west to east relative to stars, taking 27.3 days to complete cycle. What causes this monthly motion?

41 Explanations of 1 month cycle: HYPOTHESIS #1 Moon revolves around Earth at a significantly slower rate than the celestial sphere. WRONG! HYPOTHESIS #2 Moon revolves around Earth, once per month. RIGHT!

42 The Moon s orbit (not to scale) ASTR111 Lecture 2

43 Phases of the Moon The moon s phases are due to its motion relative to the Earth and Sun

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45 Observation: Both the Sun and the Moon may be eclipsed in their orbits We live in a very special place in order for this to happen

46 Solar eclipses ASTR111 Lecture 2

47 Solar eclipses Solar eclipses occur when the Moon is between the Sun and the Earth. Solar eclipses can only occur at New Moon (when the moon is dark).

48 Total Solar Eclipse: ASTR111 Lecture 2

49 Solar eclipses Only a small part of the earth observes a Total Solar Eclipse

50 Total Lunar eclipse ASTR111 Lecture 2

51 Lunar eclipse Lunar eclipses occur when Earth is between Sun and Moon. Lunar eclipses occur at Full Moon.

52 Solar vs. Lunar eclipses Total solar eclipse is visible from a narrow path (up to 270 km wide) for a short time (up to 7.5 min). Total lunar eclipse is visible from half the Earth. From Texas, next total lunar: Jan 20, 2019; next total solar: Apr 8, ryan-college-station

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