1445 Introductory Astronomy I

1. 1445 Introductory Astronomy I Chapter 1 The Night Sky and Motions of Sun, Earth and Moon R. S. Rubins Fall, 2010

2. The Geocentric Universe In the ancient idea of a geocentric universe, the Earth was assumed to be at the center of the universe. Outside the Earth, the Sun and the Moon were the most important celestial objects. We now know that the importance to us of the tiny Moon, lies only in its proximity, just 240,000 miles from the Earth. • Behind the Moon were the fixed stars, which appeared to move together around the Earth in a regular motion. Among the stars were found the planets, following irregular paths, but never straying far from the Sun’s path, which is known as the ecliptic.

3. The Planets The ancient Greeks introduced ingenious, but complicated ideas, to describe planetary motions about the Earth in a manner in keeping with the geocentric model. Their final model was that of Ptolemy (2nd century), which held sway until the Copernican revolution of the 16th century. The Earth lies at 93 million miles (or 1 astronomical unit) from the Sun, which is small distance compared to the 3 billion mile of the outermost (major) planet Neptune. Both these distances are insignificant compared to the distance the nearest star, Proxima Centauri, which is about 25 trillion miles (approximately 4 light-years) away.

4. How Many Stars? • A total of about 6000 stars can be seen by the unaided human eye, although only about half at any one time. • However, about one half of these stars that we imagine to be single are actually binary pairs; i.e. double stars, which are very close together. • Thus, without realizing it, we actually see about 9000 stars. • There are estimated to be about 200 billion (2 x 1011) stars in our galaxy, the Milky Way. • Since there are at least 50 billion galaxies in the visible universe, there should be a total of more than 10 billion trillion (1022) stars. although the vast majority cannot be seen, even with the most powerful telescopes.

5. Practical Use of Astronomy • The time to plant seeds was predicted from i. the positions of the constellations; ii. the height of the noontime Sun. • Planning sea travel often depended on the tides, which are influenced by the positions of the Moon and the Sun. • The positions of the Sun in the day and the constellations at night were used for navigation at sea. • In particular, the North Star, Polaris, was very important in navigation (in the northern hemisphere), because it closely marks the direction of due north, and its altitude in the sky gives the latitude from which it is observed.

6. Constellations • In popular usage, the term constellation is used to denote a recognizable grouping of stars. • Astronomers have redefined the constellations as 88 regions of the night sky, while referring to the groupings as asterisms. The constellation Orion popular usageastronomers usage

7. The Big Dipper as a Guide • The two “pointer stars” furthest from the handle of the Big Dipper point to Polaris(the North Star). • The next two stars point in to Regulus, the brightest star in the constellation Leo. • The pattern may appear upside-down because it rotates about Polaris.

8. The Winter Triangle The Winter Triangleconnectsthree bright stars:Betelgeuse(in Orion), Procyon(in Canis Minor) and Sirius(in Canis Major). This triangle is almost equilateral, but slightly stretched in the direction of Sirius.

9. The Summer Triangle • The Summer Triangleconnectsthree bright stars:Vega(in Lyra), Deneb(in Cygnus) and Altair(in Aquila). • This triangle is stretched in the direction of Altair.

10. Celestial Sphere 1 • The celestial sphere is an imaginary hollow sphere, with the Earth at its center, to which all the stars seen in the night sky appear to be fixed . The motion of the stars in the night sky may be visualized as a rotation of the celestial sphere from east to west about a north-south axis. • The rotation is from east to west because the stars rise in the east and set in the west. • The fixed stars are actually at widely varying distances, all more than 4 light years (25 trillion miles) away, moving relative to each other with motions that are not apparent to us. • As a result, changes in appearance of the constellations are not apparent in a human life-span.

11. Celestial Sphere 2 Know the following: North Celestial Pole South Celestial Pole Celestial Equator Declination(latitude) is measured from the Celestial Equator. Right Ascension(longitude) is measured from the VernalEquinox (see below).

12. In the Celestial Sphere picture, the stars all rotate from east to west about the line through the celestial poles. Thus, in Australia, near the Earth’s south pole, the stars all appear to rotate about the South Celestial Pole. Australian View of the South Celestial Pole

13. The Apparent Motion of the Night Sky Equator USA North Pole West West • The stars appear to move from east to west as follows: • vertically downwards at the equator (if facing West); • ii. downwards and to the right in the USA (if facing West); • iii. from left to right at the north Pole; • iv. from right to left at the South Pole.

14. Celestial Sphere and Ecliptic 3 • Geocentric view • Since the Earth is considered to be at rest at the center of the Universe, the ecliptic is defined as the annual path of the Sun around the celestial sphere. 23½ ois the angle between the ecliptic and the celestial equator. Sun moves on the ecliptic.

15. Celestial Sphere and Ecliptic 2 • In the geocentric view, the plane of the ecliptic makes an angle of 23½o, with the celestial equator.

16. The Ecliptic: Heliocentric View • In the heliocentric view, the ecliptic is defined as the path of the Earth’s orbit around the Sun. • The Earth rotates from east to west, about an axis tilted by 23½o from the normal to the ecliptic plane. 23½o North-south rotational axis. Normal to the ecliptic.

17. Equinoxes • Equinoxes(Latin “equal nights”) are those times of the year in which day and night are of roughly equal length, which occur when the Sun’s position on the ecliptic crosses the celestial equator. • The vernal equinoxoccurs on about March 21, when the Sun crosses the celestial equator heading north. • The autumnal equinoxoccurs on about September 22, when the Sun crosses the celestial equator heading south. • Between the vernal and autumnal equinoxes, the days are longer in the northern hemisphere, and the Sun is higher in the sky at mid-day. • The reverse is true for the time between the autumnal and vernal equinoxes.

18. Solstices and the Seasons • The summer solstice occurs on about June 21, when the Sun reaches the point on the ecliptic furthest north from the celestial equator. • In summer, the Sun rises in the NE and sets in the NW. • The winter solstice occurs on about December 21, when the Sun reaches the point on the ecliptic furthest south from the celestial equator. • In winter, the Sun rises in the SE and sets in the SW. • If the Earth’s rotation axis were not tilted, seasons (as we know them) would not exist, and every night would last roughly 12 hours.

19. Equinoxes, Solstices and the Seasons 3 Summer Winter The Sun’s daily path

20. The Seasons and the Earth’s Axis • The seasons result from both the 23½o tilt of the Earth’s rotation axis and its orbit about the Sun.

21. Effect of the Changing Distance of the Sun • While the Sun’s distance from the Earth varies slightly throughout the year, becoming closest on about January 3, it has no noticeable effect on the climate. • The effect of the Sun being closer in the northern winter is reduced by the fact that the southern hemisphere has a higher percentage of oceans, which reflect heat and light back into space more efficiently than do forested land masses. • If the Earth’s orbit were very elliptical (like Mercury), then this effect would be more pronounced, and if, in addition, the Earth’s axis were not tilted, then the seasons would be produced only by the varying distance of the Sun. However, in the latter case, the seasons so produced, would occur at the same time for both hemispheres.

22. The Earth’s Precessional Motion 1 • The precessional motionof the Earth’s axis is a very slow conical motion caused by the combined gravitational pulls of the Sun and the Moon. • The motion is analogous to that of a spinning top. • Calculations have shown that without the presence of the Moon, the 23½o tilt of the Earth’s rotation axis would not be maintained, with wild swings in the tilt angle being the rule.

23. The Earth’s Precessional Motion 2 • During the precessional period of about 26,000 years, the Earth’s north-south axis traces out a circle in the sky. • Presently, the celestial North Pole points to within a degree of Polaris, but in the year 14,000, it will point roughly towards Vega.

24. The Zodiac • On its apparent eastward journey around the ecliptic, the Sun appears to pass through the twelve Constellations of the Zodiac. • In 1930, astronomers added a thirteenth constellation – Ophiuchus – which the Sun passes through between December 1 and December 19 each year. • Over 2000 years ago when the pseudoscience of astrology was introduced by the famous mathematician Euclid, a person’s astrological sign was determined by where the Sun was in the Zodiac on his/her birthday. • Because of the Earth’s precessional motion, our birthdays are now one sign later than they were 2000 years ago.

25. Traveling on Spaceship Earth • Although we imagine ourselves to be at rest, the Earth takes part in the motions outlined below. • The Earth spins about its N-S axis, with a period of 1 day, and a rotational speed varies from 1650 km/hr (1030 mi/hr) at the equator to zero at the poles. • The Earth orbits the Sun with a 1 year period, and a speed of above 100,000 km/hr (60,000 mi/hr). • Our solar system orbits the center of our galaxy with a 230 million year period, and a speed slightly of about 800,000 km/h (500,000 mi/hr). • Our galaxy orbits the mass-center of the Local Groupr of galaxies, which in turn orbits the center of the Local (or Virgo) Supercluster.

26. Siderial and Synodic Periods • A siderial periodis a period measured with respect to the distant stars. • A synodic periodis the period measured from a planet (or moon). • The solar dayis the synodic day measured from Earth, which is longer than the siderial day by about 4 min. • The lunar monthis the synodic month measured from Earth, which is longer than the siderial month by approximately 2.2 days. • The tropical yearis the synodic year, measured between successive vernal equinoxes, which is shorter than the siderial year by about 20 minutes.

27. Solar and Sidereal Days • The solar dayis the average time (24 hours) between successive noon-times, as measured at 0o longitude in Greenwich, England (the prime meridian). • The sidereal day is the time (23 hours 56 min.) taken for a planet to make one complete revolution.

28. Lunar and Sidereal Months • The synodic or lunar monthis the time (approximately 29½ days) between identical phases of the moon; e.g. from full moon to full moon. • The sidereal monthis the time (approximately 27.3 days) it takes the Moon to make one full orbit (360o) around the Earth.

29. Ancient astronomers realized that the year was roughly 365¼ days long. In 47 BCE, Julius Caesar added an extra day every 4 years, thus creating leap years of 366 days. Pope Gregory XIII reformed the Julian calendar in 1582, leaving out 10 days to get the seasons back on schedule, and decreeing that only those century years divisible by 400 were to be leap years. The average Gregorian year differs by only one day in 3300 years from the tropical year. With the modification that the years 4000, 8000, 12,000 and 16,000 are not to be leap years, the Gregorian system will not have to be revised for 20,000 years. An extra second was added between Dec. 31, 2008 and Jan.1, 2009 to allow for irregularities in the Earth’s rotation. The Year and the Calendar

30. Lunar Calendars • Lunar calendars follow the Moon’s cycle, which averages 29½ days per month. • Since the year would contain only 12 x 29.5 = 354 days, an additional month was added usually every 3 years. • The Jewish calendar (now in the year 5767) is lunar, and is synchronized with the solar calendar by following the 19 year cycle, introduced by the Greek astronomer Meton in 432 BCE. • Easter has a partially lunar basis, being scheduled as the first Sunday following the first full moon on or after March 21. • The Islamic calendar is purely lunar, so that 12 months contain about 11 days fewer than a solar year. • That is why, for example, Islamic festivals, such as Ramadan begin about 11 days earlier on each subsequent year.

31. Phases of the Moon 1

32. Sky at Sunset The Moon’s position at sunset is shown for 14 evenings, beginning at thenew moonand ending at the full moon. • Note that west is to your right, which occurs if you are facing to the south, so that the Sun sets to your right.

33. Sky at Sunrise The Moon’s position at sunrise is shown for 14 evenings, beginning at thefull moonand ending at the new moon. • Note that west is to your right, which occurs if you are facing to the south, so that the Sun sets to your right.

34. PHASES OF THE MOON

35. Solar and Lunar Eclipses 1 • The plane in which the Moon orbits the Earth makes an angle of 5.2o with plane of the ecliptic. • For an eclipse to occur, the Moon must be full or new at the same time as its path crosses the ecliptic.

36. Solar and Lunar Eclipses 2 • The line of nodesis a hypothetical line joining the two points at which the Moon’s orbit crosses the ecliptic. • Eclipses occur when the line of nodes points towards the Sun.

37. The Eclipse Seasons • Eclipses are relatively rare, because for eclipses to occur, the Moon must be full or new, just as it crosses the ecliptic plane. • There are just two short periods in a year, known as the eclipse seasons, when eclipses can occur, although there is no guarantee of eclipses occurring during a particular season. • Between 2 and 5 solar eclipses can occur in a year, and a similar number of lunar eclipses. However, the total number of eclipses in a year cannot exceed 7. • It was known to ancient astronomers that the basic pattern of eclipses repeats every 18 years 11.3 days. This repetition pattern is known as the Saros cycle.

38. Solar Eclipses 1 • A solar eclipseoccurs when the Moon blocks some or all of the Sun’s light, so that the Moon’s shadow falls on the Earth. • The umbra, the central region of the Moon’s shadow, is surrounded by the penumbra . • Only in the umbra is the sunlight totally blocked, so that a total solar eclipseor an annular solar eclipseoccurs. • A total solar eclipseoccurs when the Moon is relatively close to the Earth, so that it appears large enough to totally blot out the Sun, thus allowing the faint solar corona to be seen. • An annular solar eclipseappears as a thin ring encircling the Moon’s disk when the Moon is too far from the Earth for it to totally block out the Sun.

39. Solar Eclipses 2 The umbra forms a dark spot, which is the region of the total or annulareclipse.

40. A total eclipse occurs when the Moon is close enough to block out the Sun’s surface, allowing its outermost layer – the corona – to be seen. A partial eclipse is seen from the shadow given by the Sun’s penumbra. An annular eclipse occurs when the Moon is far enough away, so that it cannot hide the Sun’s surface completely. Solar Eclipses 3

41. Total Solar Eclipse • Only during a total solar eclipse is the solar corona visible.

42. Annular Eclipse

43. Solar Eclipse Tracks 2000-2020 • The width of the track depends both on the Earth’s latitude and the distance of the Moon from the Earth during the eclipse. Saros cycle

44. Lunar Eclipses 1 A lunar eclipse occurs when the Moon enters the Earth’s shadow.

45. Lunar Eclipses 3 The Moon looks red during a total lunar eclipse for the same reason that the Sun appears reddish at sunrise and sunset, and the sky appear blue. • Sunlight is composed of all the colors of the rainbow (red, orange, yellow, green, blue, violet), and the Earth’s atmosphere preferentially scatters the blue end of this spectrum of colors. • The scattered blue light gives the sky its color, while the missing blue end of the spectrum makes the Sun appear yellow during the day and red at sunrise and sunset, when the Sun’s rays take a longer path through the atmosphere to reach us.

46. Lunar Eclipses 2 • The Moon appears red in a total lunar eclipse because of the preferential scattering by the Earth’s atmosphere of the blue end of the spectrum of colors in sunlight. • As a result, more of the Sun’s red light reaches the Moon.

47. Lunar Eclipse Over Dallas