This Set of Slides These slides cover some more information about time, the phases of the moon, and solar and lunar eclipses. Units covered: 7 and 8.

What Time Is It? There are many ways to measure time on Earth –Sunrise to sunrise Problem – seasons change sunrise times. –The time between successive crossings of the meridian by the Sun (Solar Day) Problem – inaccuracies due to cloud cover. Every town has it’s own noon!

Sidereal Time One solar day is 24 hours. It takes 24 hours for the sun to return to the same (overhead) spot in the sky on successive days. One sidereal day is 23 hours, 56 minutes. –Represents the time it takes for the background stars to return to the same position. –The time for the Earth to rotate through 360 degrees. –Why are they different? The Earth’s rotation!

Length of Daylight Hours The number of daylight hours a place has depends on that place’s latitude. –Regions closer to the northern pole get more daylight hours during the summer, and less in winter. –Within the Arctic Circle (higher than 66.5 degrees latitude), there are some summer days where the Sun never sets! In the dead of winter, it will never rise. –(Same thing occurs at the south pole as well of course.) –Regions close to the equator get close to 12 hours of sunlight everyday all year long.

Time Zones The globe is divided into 24 time zones, designed such that local noon roughly corresponds to the time when the sun is highest in the sky. If it is noon on the Prime Meridian in Greenwich, UK, it is midnight on the opposite side of the world. This midnight line is called the International Date Line. Need for time zones was driven by commerce, “rapid” transportation, and long distance communication.

The Phases (Faces?) of the Moon As the Moon moves around the Earth in its 29.5 day cycle, one half of its surface is always lit by the sun. From Earth, we see only the illuminated portion, giving the appearance of phases of the Moon. –Full Moon: The Earth is between the Moon and the Sun, so we see all of the illuminated surface. –New Moon: The Moon is between the Earth and the Sun, so we see none of the illuminated surface.

Solar Eclipses At New Moon, the Moon is between the Earth and the Sun. Sometimes, the alignment is just right, allowing the Moon to block the light from the Sun, creating an eclipse. A solar eclipse seen from space

Solar Eclipse – the Shadow of the Moon In a solar eclipse, the Moon casts a shadow on the surface of the Earth. People within the shadow see the eclipse, and those outside the shadow do not. The Moon’s umbra is the darkest part of the shadow, directly behind the body of the Moon. Within the umbra, the Sun appears completely eclipsed (total eclipse). The penumbra of the Moon (not shown in figure) is the part of the shadow where the light from the Sun is only partially blocked (partial eclipse). A solar eclipse seen from space

Lunar Eclipses As the Moon passes behind the Earth, the Earth can cast a shadow on the surface of the Moon, creating a lunar eclipse. The reddish glow of a fully eclipsed Moon is light that has been refracted through the Earth’s atmosphere and bounced back to Earth – it is, in essence, the light of every sunrise and sunset on Earth reflected off the Moon.

Lunar Eclipse – the Shadow of the Earth In a lunar eclipse, the Earth casts a shadow on the surface of the Moon. In its orbit, the Moon passes through the penumbra and umbra of the Earth The penumbra of the Earth is the part of the shadow where the light from the Sun is only partially blocked. The Moon dims a little as it passes into the penumbra. The Earth’s umbra is the darkest part of the shadow, directly behind the body of the Earth. If the whole of the Moon moves into the umbra, its surface becomes very dark though red. This is a total lunar eclipse.

Why don’t eclipses happen all the time? As a consequence of this orbit “tilt”, most of the time, the Moon’s shadow misses the Earth, or the Earth’s shadow misses the Moon. There are only a couple times of the year when eclipses are possible. In order for an eclipse to occur, the Moon must lie directly between the Earth and the Sun (solar eclipse), or the Earth must lie directly between the Moon and the Sun (lunar eclipse). The orbit of the Moon around the earth is inclined slightly to the plane of the ecliptic (the plane in which the Earth’s orbit lies).

Everything Must be Just Right For an Eclipse For an eclipse to occur, the Moon must be crossing the ecliptic at the same time it passes either in front of (solar eclipse) or behind (lunar eclipse) the Earth (positions B&D). Otherwise, no eclipses are possible (positions A&C and everywhere else.)

Eclipses in 2010 January 15, Annular Solar Eclipse. (Middle of ocean between India and Africa.) June 26, Partial Lunar Eclipse. (U.S. West Coast at moon set. South Pacific, NE of Australia.) July 11, Total Solar Eclipse. (South Pacific, East of Chile.) December 21, Total Lunar Eclipse. (North America)