Homework #2 45.635 degrees is how many degrees, arcminutes, and arcseconds? How many degrees, arcminutes, and arcseconds does the moon move across the sky in one hour? (the lunar day is 24 hours and 48 minutes long) The moons diameter is about 30 arcminutes, so find out how long it takes for the moon to travel its diameter.

The Daily Motion As the Earth rotates, the sky appears to us to rotate in the opposite direction. The sky appears to rotate around the N (or S) celestial poles. If you are standing at the poles, nothing rises or sets. If you are standing at the equator, everything rises & sets 90 to the horizon.

The Daily Motion The altitude of the celestial pole = [your latitude]. All stars at an angle < [your latitude] away from: your celestial pole never set. (circumpolar) the other celestial pole are never seen by you. Other stars, (& Sun, Moon, planets) rise in East and set in West at an angle = [90  your latitude].

The Daily Motion daily circles --- CCW looking north, CW looking south Höküpaÿa Newa

Time Exposure Photograph: • Estimate the exposure time • Which direction did stars move?

Gemini Telescope

Hawai`i Chile

Time Exposure Photograph: • Estimate the exposure time • Which direction did stars move? l

Time Exposure Photograph: • Estimate the exposure time • Which direction did stars move?

Annual Motion As the Earth orbits the Sun, the Sun appears to move eastward with respect to the stars. The Sun circles the celestial sphere once every year.

Annual Motion ecliptic equinox solstice zodiac the apparent path of the Sun through the sky equinox where the ecliptic intersects the celestial equator solstice where the ecliptic is farthest from the celestial equator zodiac the constellations which lie along the ecliptic

Annual Motion The Earth’s axis is tilted 23.5° from being perpendicular to the ecliptic plane. Therefore, the celestial equator is tilted 23.5° to the ecliptic. As seen from Earth, the Sun spends 6 months north of the celestial equator and 6 months south of the celestial equator.

The Cause of the Seasons Summer Winter Seasons are caused by the Earth’s axis tilt, not the distance from the Earth to the Sun!

The Cause of the Seasons

Seasonal Change in Sun’s Altitude The “Figure 8” shows Sun at same time each day over a year.

When is summer? The solstice which occurs around June 21 is considered the first day of summer. It takes time for the more direct sunlight to heat up the land and water. Therefore, July & August are typically hotter than June.

Why doesn’t orbital distance from the Sun matter? Small variation for Earth — about 3% (but orbit distance does matter for some other planets, notably Mars and Pluto). Surprisingly, seasons are more extreme in N. hemisphere, even though Earth is closer to Sun in S. hemisphere summer (and farther in S. hemisphere winter) — because of land/ocean distribution

Precession of the Equinoxes The Earth’s axis precesses (wobbles) like a top, once about every 26,000 years. Precession changes the positions in the sky of the celestial poles and the equinoxes. Polaris won't always be the north star. The spring equinox, seen by ancient Greeks in Aries, moves westward and is now in Pisces!

Phases of Moon Half of the Moon is illuminated by the Sun and half is dark. We see a changing combination of the bright and dark faces as the Moon orbits Earth. You may want to do an in-class demonstration of phases by darkening the room, using a lamp to represent the Sun, and giving each student a Styrofoam ball to represent the Moon. If you lamp is bright enough, the students can remain in their seats and watch the phases as they move the ball around their heads. How to Simulate Lunar Phases

Lunar Motion Phases of the Moon’s 29.5 day cycle waxing waning new Muku Külua ÿOlekükahi Huna Hoku Käloakükahi Lono new crescent first quarter gibbous full last quarter waxing waning

hoÿonui Hilo Hoaka Kükahi Külua Küpau ÿOlekükahi ÿOlekülua Kükolu ÿOlekükolu ÿOleküpau

poepoe Huna Möhalu Hua Hoku MäheaIani Kulu Laÿaukükahi Laÿaukülua Akua Hoku MäheaIani Kulu Laÿaukükahi poepoe Laÿaukülua Laÿauküpau

ÿemi ÿOlekükahi ÿOlekülua ÿOleküpau Käloakükahi Käloakülua Käloapau Käne Lono ÿemi Mauli Muku

We see only one side of the Moon Synchronous rotation: The Moon rotates exactly once with each orbit. This is why only one side is visible from Earth. Use this tool from the Phases of the Moon tutorial to explain rise and set times for the Moon at various phases. As usual, please encourage your students to try the tutorial for themselves.

Thought Question First quarter Waxing gibbous Third quarter Half moon It’s 9 A.M. You look up in the sky and see a moon with half its face bright and half dark. What phase is it? First quarter Waxing gibbous Third quarter Half moon This will check whether students have grasped the key ideas about rise and set times.

Thought Question First quarter Waxing gibbous Third quarter Half moon It’s 9 A.M. You look up in the sky and see a moon with half its face bright and half dark. What phase is it? First quarter Waxing gibbous Third quarter Half moon If anyone chose “half moon,” remind them that there is no phase with that name… (and that first and third quarter refer to how far through the cycle of phases we are…)

What causes eclipses? The Earth and Moon cast shadows. When either passes through the other’s shadow, we have an eclipse. This slide starts our discussion of eclipses. Use the figure to explain the umbra/penumbra shadows.

Lunar Eclipse Lunar Eclipse This interactive tool goes through lunar eclipses. Use it instead of or in addition to the earlier slides on eclipses. Lunar Eclipse

Solar Eclipse Evolution of a Total Solar Eclipse This interactive tool goes through the solar eclipses. Use it instead of or in addition to the earlier slides on eclipses. Evolution of a Total Solar Eclipse

When can eclipses occur? Lunar eclipses can occur only at full moon. Lunar eclipses can be penumbral, partial, or total. Use the interactive figure to show the conditions for the 3 types of lunar eclipse.

When can eclipses occur? Solar eclipses can occur only at new moon. Solar eclipses can be partial, total, or annular. Use the interactive figure to show the conditions for the 3 types of solar eclipse.

Why don’t we have an eclipse at every new and full moon? The Moon’s orbit is tilted 5° to ecliptic plane. So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon. Use this pond analogy to explain what we mean by nodes and how we get 2 eclipse seasons each year (roughly). Note: You may wish to demonstrate the Moon’s orbit and eclipse conditions as follows. Keep a model “Sun” on a table in the center of the lecture area; have your left fist represent the Earth, and hold a ball in the other hand to represent the Moon. Then you can show how the Moon orbits your “fist” at an inclination to the ecliptic plane, explaining the meaning of the nodes. You can also show eclipse seasons by “doing” the Moon’s orbit (with fixed nodes) as you walk around your model Sun: the students will see that eclipses are possible only during two periods each year. If you then add in precession of the nodes, students can see why eclipse seasons occur slightly more often than every 6 months.

Summary: Two conditions must be met to have an eclipse: It must be a full moon (for a lunar eclipse) or a new moon (for a solar eclipse). 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).

Eclipse Predictions Eclipses recur in the approx. 18 yr, 11 1/3 day saros cycle But even then, eclipse location and type (e.g., partial, total) may vary

Photo by Olaf Menkens (1999 Mauna Loa)