Astronomy 110 Announcements: Quick review from yesterday: • Seasons caused by the Earth’s • Reading assignment for tomorrow: axis tilt, not by distance from the Sun. pp. 53 – 67 in ECP • (in the Northern hemisphere) on the summer solstice, the sun rises N of E and sets N of • Reminder: Homework #1 due next W. The sun follows it highest Tuesday—come see me with any questions path along the sky on this date (caveat for locations between the tropics—highest path of sun is somewhere between the equinox and the solstice). • On the equinoxes, the sun rises due E and sets due W. It is above and below the horizon for 12 hours each.

A few definitions: Motion of the Moon: recap. from yesterday

• Ecliptic – the path that the Sun appears to travel throughout • The moon orbits the Earth the year (due to the Earth’s revolution about the sun) as once every 27.3 days viewed against the backdrop of more distant stars • The moon’s orbit follows the Right Hand Rule: • Zodiac – the constellations that lie on the ecliptic plane. i.e., – Note: The Earth’s rotation the constellations that the Sun appears to travel through. and revolution about the sun These are the same constellations used in astrology. follow this same rule—results in counter- clockwise rotation • Sidereal – motion relative to the stars – Ex: sidereal day: the time it takes for the Earth to complete 1 rotation with respect to the stars = 23hr 56m Motion of the Moon: (recap.) Why do we see phases of the Moon? • Orbital planes of Moon and Sun are not quite aligned (off by about 5°) • The moon rises in the East and sets in the West, due • Half the Moon to the Earth’s counter-clockwise rotation illuminated by Sun • Because the Moon orbits the Earth, it appears to and half dark move at a different rate on the sky than the stars. • We see some – Moon moves eastward relative to stars. combination of the – Every 27.3 days it moves all the way through the zodiac bright and dark • Therefore: faces – The Moon moves about 15 degrees eastward every day, or about 0.5 degrees per hour (about its angular size on the sky). – Moon rises about 1 hour later each night.

Phases of the Moon: 29.5-day cycle Thought Question It’s 9 am. You look up in the sky and see a new moon with half its face bright and half dark. What phase is it? crescent first quarter waxing A. First quarter • Moon visible in afternoon/evening. B. Waxing gibbous gibbous } • Gets “fuller” and rises later each day. C. Third quarter full D. Half moon gibbous waning last quarter • Moon visible in late night/morning. • Gets “less” and sets later each day. crescent } Question: It’s 9 am. You look up in the sky and see a How long does it take to see one complete moon with half its face bright and half dark. cycle of lunar phases? What phase is it? (how many days between each full moon?) A. First quarter A. Less than 27.3 days B. Waxing gibbous B. Exactly 27.3 days C. Third quarter C. More than 27.3 days — Why?

D. Half moon • The cycle of lunar phases takes about 29.5 days (“synodic month”) • Cycle longer than the Moon’s rotation period (27.3 days) because of the Earth’s motion around the Sun—the Moon has to “catch up”.

What about the far side of the moon? What about the far side of the moon?

• Due to “tidal locking”, a.k.a. “orbit-spin resonance”, a.k.a. “synchronous” orbit, the moon always shows the same face to the Earth. This means that it orbits the Earth at the same rate as it rotates on its axis. – If the Earth was tidally locked with the Sun, one side would always face the Sun, getting scorching hot, while the other side would be dark and cold • The Moon is not tidally locked with the sun – Every surface of the moon eventually gets some sunlight. The far side is only completely dark when we see a “full moon” from Earth. Therefore, the “dark side” of the moon does not make any sense when used to imply the far side of the moon. What causes eclipses? When can eclipses occur?

• The Earth and Moon cast shadows. • Lunar eclipses can occur only at full moon. • When either passes through the other’s shadow, we • Lunar eclipses can be penumbral, partial, or total. have an eclipse.

Why don’t we have an eclipse at every new and full When can eclipses occur? moon? – The Moon’s orbit is tilted 5° to ecliptic plane… • Solar eclipses can occur only at new moon. – So we have about two eclipse seasons each year, with a lunar • Solar eclipses can be partial, total, or annular. eclipse at new moon and solar eclipse at full moon. Summary: Two conditions must be met to Predicting Eclipses have an eclipse: • Eclipses recur with the 18 yr, 11 1/3 day saros cycle, but type (e.g., partial, total) and location 1. It must be full moon (for a lunar eclipse) or new moon (for a solar eclipse). may vary. AND 2. The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes).

2.1 Patterns in the Night Sky What are constellations? Our goals for learning: A constellation is a • What are constellations? region of the sky (a pattern of stars is • How do we locate objects in the sky? called an asterism). • Why do stars rise and set? 88 constellations • Why don’t we see the same constellations fill the entire sky. throughout the year? Stars within the constellations may be quite far away from each other—they only appear to be close together due to projection effects (the stars are so far away that we can’t judge distances) The Celestial Sphere The Milky Way A band of light making a circle around the celestial sphere.

What is it? Our view into the plane of our galaxy.