1. 31 Aug 2000ASTR103, GMU, Dr. Correll1 Ch 1--Discovering the night sky

2. 31 Aug 2000ASTR103, GMU, Dr. Correll2 Outline Patterns in the stars Earthly cycles Phases of the Moon Eclipses

3. 31 Aug 2000ASTR103, GMU, Dr. Correll3 Patterns of Stars Only about 6000 stars are visible to the eye –other than the milky way, to be discusses later Skywatchers have always imagined groups of stars depicting mythical, natural, or heroic beings These constellations, or star groupings, are convenient ways to talk about areas of the sky

4. 31 Aug 2000ASTR103, GMU, Dr. Correll4 Constellations The Constellation Orion

5. 31 Aug 2000ASTR103, GMU, Dr. Correll5 Navigating the sky To talk about where stars are in the night sky we need to define a reference system –How can we do this? –Let’s consider the natural motions of the earth!

6. 31 Aug 2000ASTR103, GMU, Dr. Correll6 The Celestial Sphere

7. 31 Aug 2000ASTR103, GMU, Dr. Correll7 Navigating the sky North and South Celestial Poles--defined by the Earth’s axis of rotation Celestial Equator--a flat plane through the middle of the Earth and at right angles to the rotation axis Right Ascension--angle along the equator measured from the Vernal Equinox Declination--angle measured above or below the equator

8. 31 Aug 2000ASTR103, GMU, Dr. Correll8 Outline Patterns in the stars Earthly cycles Phases of the Moon Eclipses

9. 31 Aug 2000ASTR103, GMU, Dr. Correll9 Motion of the night sky Diurnal motion--daily motion of the stars –some rise and set –some always visible near the celestial pole (circumpolar) –some never seen near the opposite celestial pole

10. 31 Aug 2000ASTR103, GMU, Dr. Correll10 Daily Motion of the Sun The sun rises and sets with a period of 24 hours –That is, the daily cycle of the sun in the sky was divided into 24 equal time periods. –This motion defines a measure of time call the “day” Since everyone wants the noon to be the middle of the day, we’ve developed time zones We’ve also marked a grid of Latitude and Longitude to describe positions on the Earth –similar to Right Ascension/Declination, but fixed to the Earth (Prime Meridian in Greenwich England) rather than the distant stars

11. 31 Aug 2000ASTR103, GMU, Dr. Correll11 Daily Motion of the Sun

12. 31 Aug 2000ASTR103, GMU, Dr. Correll12 Sun/Earth motion The year is defined by the annual cycle of the apparent motion of the Sun in the sky over the course of the seasons –At higher northern latitude during summer –At lower southern latitude during winter Approximately 365.25 days per year –We have a leap year (Feb 29--to give 366 days) every four years--Julian calendar (Julius Ceasar) –Modified by Pope Gregory, Oct 5/15, 1582 This keeps the seasons at the same place in the calendar--very important for agricultural planning and religious festivals!

13. 31 Aug 2000ASTR103, GMU, Dr. Correll13 Sun/Earth motion

14. 31 Aug 2000ASTR103, GMU, Dr. Correll14 Daily Cycle Errata Siderial day--time for Earth to make one rotation compared to the fixed stars –23 hours 56 minutes Synodic day--time for the Earth to complete one full cycle of phases--that is, relative to the Sun-- high noon to high noon –24 hours

15. 31 Aug 2000ASTR103, GMU, Dr. Correll15 Solar and Sidereal Days Sun Earth’ Orbit Day 1 Day 2 1o1o1o1o 1o1o1o1o

16. 31 Aug 2000ASTR103, GMU, Dr. Correll16 Rotation/Revolution As the Earth revolves about the Sun, it defines a reference plane called the ecliptic! The ecliptic and the celestial equator are not lined up--this is because the axis of Earth’s rotation is tilted 23.5 deg from it’s axis of revolution about the Sun! This causes the seasons!

17. 31 Aug 2000ASTR103, GMU, Dr. Correll17 The Four Seasons

18. 31 Aug 2000ASTR103, GMU, Dr. Correll18 The Four Seasons

19. 31 Aug 2000ASTR103, GMU, Dr. Correll19 Astrological aside--constellation of the Zodiac Scorpius Cancer Aries Libra Sagittarius Capricornus Taurus GeminiLeo January 1 March 1 Virgo Pisces Aquarius Ecliptic

20. 31 Aug 2000ASTR103, GMU, Dr. Correll20 Precession of Earth’s axis The tilt of the Earth’s axis precesses with a period of 26,000 years. Due to the gravitational pull of the the Sun and Moon on the Earth’s equatorial bulge

21. 31 Aug 2000ASTR103, GMU, Dr. Correll21 Precession of Earth’s axis Location of the Pole Star changes over the millenia Precession of the Equinoxes also results

22. 31 Aug 2000ASTR103, GMU, Dr. Correll22 Outline Patterns in the stars Earthly cycles Phases of the Moon Eclipses

23. 31 Aug 2000ASTR103, GMU, Dr. Correll23 Phases of the moon Waxing--growing Waning--diminishing New--obscured or small sliver Cresent Quarter--a half circle Gibbous--more than half Full

24. 31 Aug 2000ASTR103, GMU, Dr. Correll24 Lunar Cycle Siderial month--time for Moon to make one orbit about the Earth compared to the fixed stars –27.3 days Synodic month--time for the Moon to complete one full cycle of phases--that is, relative to the Sun –29 days Plane of Moon’s orbit about the Earth is inclined about 5 deg from the Ecliptic

25. 31 Aug 2000ASTR103, GMU, Dr. Correll25 Outline Patterns in the stars Earthly cycles Phases of the Moon Eclipses

26. 31 Aug 2000ASTR103, GMU, Dr. Correll26 Eclipses Eclipse--when one body shadows another Lunar Eclipse--Moon enters Earth’s shadow (moon hidden) Solar Eclipse--Earth passes under Moon’s shadow (Sun hidden) Umbra--full shadow Penumbra--partial shadow

27. 31 Aug 2000ASTR103, GMU, Dr. Correll27 Eclipses

28. 31 Aug 2000ASTR103, GMU, Dr. Correll28 Lunar Eclipses

29. 31 Aug 2000ASTR103, GMU, Dr. Correll29 Lunar Eclipses Penumbral--Moon passes through Earth’s penumbra only Partial--Only part of Moon passes through Earth’s umbra Total--Moon passes entirely through Earth’s umbra

30. 31 Aug 2000ASTR103, GMU, Dr. Correll30 Solar Eclipses

31. 31 Aug 2000ASTR103, GMU, Dr. Correll31 Solar Eclipses Sun and Moon about the same angular size--0.5 deg –Total Eclipse--Moon completely obscures the Sun –Annular Eclipse--Moon at greatest distance from Earth and is smaller in angular size than Sun Partial Eclipse--Earth enters Moon’s penumbra only