Ch 1--Discovering the night sky

1. Ch 1--Discovering the night sky ASTR103, GMU, Dr. Correll

2. Outline • Patterns in the stars • Earthly cycles • Phases of the Moon • Eclipses ASTR103, GMU, Dr. Correll

3. 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 ASTR103, GMU, Dr. Correll

4. Constellations The Constellation Orion ASTR103, GMU, Dr. Correll

5. 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! ASTR103, GMU, Dr. Correll

6. The Celestial Sphere ASTR103, GMU, Dr. Correll

7. 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 ASTR103, GMU, Dr. Correll

8. Outline • Patterns in the stars • Earthly cycles • Phases of the Moon • Eclipses ASTR103, GMU, Dr. Correll

9. 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 ASTR103, GMU, Dr. Correll

10. 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 ASTR103, GMU, Dr. Correll

11. Daily Motion of the Sun ASTR103, GMU, Dr. Correll

12. 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! ASTR103, GMU, Dr. Correll

13. Sun/Earth motion ASTR103, GMU, Dr. Correll

14. 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 ASTR103, GMU, Dr. Correll

15. Solar and Sidereal Days Earth’ Orbit 1o Day 2 1o Day 1 Sun ASTR103, GMU, Dr. Correll

16. 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! ASTR103, GMU, Dr. Correll

17. The Four Seasons ASTR103, GMU, Dr. Correll

18. The Four Seasons ASTR103, GMU, Dr. Correll

19. Capricornus Sagittarius Aquarius Ecliptic Scorpius Pisces Libra Aries January 1 March 1 Taurus Virgo Gemini Leo Cancer Astrological aside--constellation of the Zodiac ASTR103, GMU, Dr. Correll

20. 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 ASTR103, GMU, Dr. Correll

21. Precession of Earth’s axis • Location of the Pole Star changes over the millenia • Precession of the Equinoxes also results ASTR103, GMU, Dr. Correll

22. Outline • Patterns in the stars • Earthly cycles • Phases of the Moon • Eclipses ASTR103, GMU, Dr. Correll

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

24. 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 ASTR103, GMU, Dr. Correll

25. Outline • Patterns in the stars • Earthly cycles • Phases of the Moon • Eclipses ASTR103, GMU, Dr. Correll

26. 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 ASTR103, GMU, Dr. Correll

27. Eclipses ASTR103, GMU, Dr. Correll

28. Lunar Eclipses ASTR103, GMU, Dr. Correll

29. 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 ASTR103, GMU, Dr. Correll

30. Solar Eclipses ASTR103, GMU, Dr. Correll

31. 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 ASTR103, GMU, Dr. Correll