Unit 17

In this module we find out all about our solar system and stars and galaxies. We learn how we have seasons, and why a year is 365 days (and a quarter!) and lots of interesting stuff.

Have you ever wondered why a day is 24 hours long, or a year 365 days or what a leap year is about? Well no need to any more. The answers are here. Read on.

We live on the planet Earth which is a small rocky planet that rotates around a star which we call our sun. Earth spins on its axis and one full spin takes 24 hours ( a day). When the bit of the earth we are standing on spins away from the sun we say it is night because it is dark. When we are facing the sun it is light and we call that the day. Easy so far!

The earth revolves around the sun and when it has gone all the way around and is back where it started we say a year has passed. This takes 365 (and a quarter) days. We add the quarter days together and every four years (a leap year) we add an extra day to our calendar!

Beneath the crust, the Earth is a sphere of hot rock and metal. By studying the records of earthquake waves, scientists have learned that the inside of the Earth is divided into three parts: the mantle, the outer core, and the inner core. The mantle is a thick layer of rock below the crust. It goes down about 1,800 miles (2,900 kilometers). The rock in the mantle is made of silicon, oxygen, aluminum, iron, and magnesium. The uppermost part of the mantle has a temperature of about 1600 degrees F (870 degrees C). The temperature gradually increases to about 8000 degrees F (4400 degrees C) in the deepest part of the mantle.

The outer core begins about 1,800 miles (2,900 kilometers) below the Earth's surface. Scientists believe the outer core is about 1,400 miles (2,250 kilometers) thick and consists mainly of melted iron, combined with a small amount of some other elements, most likely oxygen or sulfur. Its temperature ranges from about 8000 degrees F (4400 degrees C) in the uppermost parts to about 11,000 degrees F (6100 degrees C) in the deepest parts.

The changing seasons are caused by the changing position of the Earth in relation to the sun. Astronomers can tell exactly from the Earth's motion around the sun when one season ends and the next one begins. The dates used for the first day of each season mark the beginning of the astronomical seasons. The beginning and end of the climatic seasons vary from these dates from place to place and from year to year. The temperature and weather do not change instantly in response to the changing position of the Earth in relation to the sun. The warmest and coldest weather generally occurs several weeks after the beginning of the summer and winter astronomical seasons.

Heat that has been retained by the oceans plays an important role in producing this delayed response. The seasons keep changing because the tilt of the Earth's axis never changes while the Earth circles the sun. One way to understand this is to picture which way the tilt of the axis causes the North Pole to slant at different times of the year.
When the North Pole slants toward the sun, the Northern Hemisphere receives the most sunlight and it is summer there. When the pole slants away from the sun, the Northern Hemisphere receives the least sunlight and it is winter. Spring begins when the pole starts to slant toward the sun, and autumn begins when the pole starts to slant away again.

This is a photograph showing the actual phases of the moon. Cick on it for a better look. Note how the moon shows the same face to the Earth at all times. You can see the moon really well with binoculars. Try it!

	New Moon - The Moon's unilluminated side is facing the Earth. The Moon is not visible (except during a solar eclipse).
	Waxing Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.
	First Quarter - One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.
	Waxing Gibbous - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.
	Full Moon - The Moon's illuminated side is facing the Earth. The Moon appears to be completely illuminated by direct sunlight.
	Waning Gibbous - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.
	Last Quarter - One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.
	Waning Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.

The following waning crescent is a New Moon, beginning a repetition of the complete phase cycle of 29.5 days average duration. The time in days counted from the time of New Moon is called the Moon's "age". Each complete cycle of phases is called a "lunation"

***** The Islamic calendar is based on lunar months, which begin when the thin crescent Moon is actually sighted in the western sky after sunset within a day or so after New Moon. The 12 months of the Islamic calendar are: Muharram, Safar, Rabi'a I, Rabi'a II, Jumada I, Jumada II, Rajab, Sha'ban, Ramadan, Shawwal, Dhu al-Q'adah, Dhu al-Hijjah. Since 12 lunar months are, on average, 11 days shorter than the calendar we use (Gregorian), the Islamic year shifts earlier in each year by about this amount. The count of years for the Islamic calendar begins in 622.