Astronomy Picture of the Day

1. Astronomy Picture of the Day

2. http://panda.unm.edu/courses/tessier/astro101 • Today: • Chapter 1 (Moon phases and Eclipses, Birth of modern astronomy) • Start using CPS system today: • Test comprehension • Give an idea of possible test questions

3. CPS Question • That Polaris will not always be the North star is due to _______. • A) the sidereal day being shorter than the solar day. • B) precession shifting the celestial poles in the sky. • C) the moon following the ecliptic, instead of the equator. • D) the Earth's period being slightly longer than 365 days. • E) the Solar wind blowing the Earth away from the sun.

4. CPS Question • The parallax angle becomes _______. • A) larger as the distance to the foreground object increases. • B) larger as the baseline increases. • C) smaller as the distance to the foreground object increases. • D) Both A and B are correct. • E) Both B and C are correct.

5. CPS Question • If the tilt of the Earth's axis of rotation were made perpendicular to the plane of the ecliptic then its _______ would disappear. • A) magnetic field • B) seasons • C) eclipses • D) phases • E) Both C and D are correct.

6. Chapter 1 The Motion of the Moon and the Birth of Modern Science The Moon has a cycle of "phases", which lasts about 29 days. Half of the Moon's surface is lit by the Sun. During this cycle, we see different fractions of the sunlit side. Which way is the Sun here?

8. Cycle of phases slightly longer than time it takes Moon to do a complete orbit around Earth. Orbit time or "sidereal month" Cycle of phases or "synodic month" 27.3 days 29.5 days

9. Questions • What sort of alignment of the Earth, Moon, and Sun is required for a lunar eclipse? • How about for a solar eclipse?

10. Eclipses Lunar Eclipse (around 100 minutes) When the Earth passes directly between the Sun and the Moon. Sun Earth Moon Solar Eclipse (no more than about 8 minutes) When the Moon passes directly between the Sun and the Earth. Earth Moon Sun

11. Lunar Eclipse

12. Solar Eclipses

13. Solar Eclipses Diamond ring effect - just before or after total Total Partial Annular - why do these occur?

14. During which phase(s) can a lunar eclipse occur? What about a solar eclipse? Why don't eclipses occur every month?

15. Moon's orbit tilted compared to Earth-Sun orbital plane: Sun Moon Earth 5.2o Side view Moon's orbit slightly elliptical: Moon Distance varies by ~12% Earth Top view, exaggerated ellipse

16. Lunar (solar) eclipses occur when a full (new) moon coincides with the intersection of the moon’s orbit with the ecliptic. The plane of the Moon's orbit precesses, so that the eclipse season occurs about 19 days earlier each year. (DEMO- Sun, Earth, Moon model)

17. CPS Question • The full moon has _____ angular diameter as the Sun. • A) a much larger • B) a much smaller • C) about the same

18. CPS Question • If you are in the moon's umbral shadow, then you will witness _____. • A) nighttime • B) a total solar eclipse • C) a total lunar eclipse • D) a full moon • E) a new moon

19. From Aristotle to Newton The history of the Solar System (and the universe to some extent) from ancient Greek times through to the beginnings of modern physics. Where do we put the other planets in our picture of the solar system? What are some reasons that the geocentric model of the universe seems to make intuitive sense?

20. Geocentric vs. Heliocentric Models of the Solar System - Ancient Greek astronomers knew of Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. - Aristotle vs. Aristarchus (3rd century B.C.) Aristotle: Sun, Moon, Planets and Stars rotate around fixed Earth. Aristarchus: Used geometry of eclipses to show Sun bigger than Earth (and Moon smaller), so guessed that Earth orbits the Sun. Also guessed Earth spins on its axis once a day => apparent motion of stars. Aristotle: But there's no wind or stellar parallax. Doesn’t “feel” like we are moving. Actually orbiting sun at 70,000 mph! • Major problem with Aristotle's "Geocentric" model: "Explaining Retrograde motion of the planets"

21. Planets generally move in one direction relative to the stars, but sometimes they appear to loop back. This is "retrograde motion". Loops are called "epicycles". Apparent motion of Mars against "fixed" stars Mars 7 July * Earth * 7 6 6 5 * 3 * 4 4 * 5 3 2 2 * 1 1 January

22. Geocentric Model(Earth Centered) • Parameters: Sizes of (and speeds along) epicycles and deferents • Fairly good agreement with retrograde motion of planets • Some predictive power • More precise measurements showed errors

23. To account for retrograde motion in the geocentric model, you must attribute epicycles to actual motions of planets. Ptolemy's geocentric model (A.D. 140)

24. Heliocentric Model • Rediscovered by Copernicus in 16th century. • Much simpler. Explains retrograde motion. • Not generally accepted then. • Put Sun at center of everything. • Circular orbits. Almost got rid of epicycles. • Opposed by Catholic Church Copernicus 1473-1543

25. Illustration from Copernicus' work showing the heliocentric model. • Insistence on circular orbits made epicycles necessary. • Main motivation was simplicity.

26. Galileo (1564-1642) Built his own telescope. Discovered four moons orbiting Jupiter. What does this suggest? Discovered sunspots. What might we infer about the Sun from these observations? Discovered phases of Venus.

27. Galileo (1564-1642) Built his own telescope. Discovered four moons orbiting Jupiter => Earth is not center of all things! Discovered sunspots. Deduced Sun rotated on its axis. Surface features => Sun not a “perfect” object. Discovered phases of Venus, inconsistent with geocentric model.

28. Heliocentric model easily accounts for phases of Venus

29. Geocentric model fails to account for phases of Venus