Introduction Introduction to Astronomyto Astronomy

March 24, 2009March 24, 2009

Days and YearsDays and Years

The ancient Egyptians were among the The ancient Egyptians were among the first people to study the stars. first people to study the stars.

The study of the moon, stars and other The study of the moon, stars and other objects in space is called objects in space is called astronomyastronomy..

Ancient astronomers also studied the Ancient astronomers also studied the movements of the sun and the moon as movements of the sun and the moon as they appeared to travel across the sky.they appeared to travel across the sky.

Days and YearsDays and Years

It seemed to earlier It seemed to earlier astronomers that the astronomers that the Earth was standing still Earth was standing still and the sun and the and the sun and the moon were moving.moon were moving.

Actually, the sun and Actually, the sun and moon seem to move moon seem to move across the sky each day across the sky each day mainly because Earth is mainly because Earth is rotating on its rotating on its axisaxis..

RotationRotation The imaginary line that The imaginary line that

passes through Earth’s passes through Earth’s center and the North and center and the North and South Poles is called South Poles is called Earth’s Earth’s axisaxis..

The spinning of the Earth The spinning of the Earth on this axis is called on this axis is called rotationrotation. A point on the . A point on the equator rotates at about equator rotates at about 1,600 kilometers per 1,600 kilometers per hour. Even most hour. Even most commercial jet planes commercial jet planes can’t fly this fast!can’t fly this fast!

Day and NightDay and Night KNOW THIS!KNOW THIS!

Earth’s rotation on its axis Earth’s rotation on its axis causes day and night.causes day and night.

As Earth rotates As Earth rotates eastward, the sun eastward, the sun appears to move appears to move westward across the sky.westward across the sky.

It is day on the side of It is day on the side of Earth facing the sun.Earth facing the sun.

As Earth continues to turn As Earth continues to turn to the east, the sun to the east, the sun appears to set in the appears to set in the west, so it is night there.west, so it is night there.

RotationRotation

To rotate around the axis, Earth takes To rotate around the axis, Earth takes about 24 hours to do so.about 24 hours to do so.

As you know, each 24-hour time is a day.As you know, each 24-hour time is a day.

RevolutionRevolution In addition to rotating on In addition to rotating on

its axis, Earth travels its axis, Earth travels around the sun. The around the sun. The movement of one object movement of one object around another object is around another object is called called revolutionrevolution..

One complete revolution One complete revolution around the sun is called a around the sun is called a yearyear..

Earth’s path as it revolves Earth’s path as it revolves around the sun is called around the sun is called its its orbitorbit..

Check PointCheck Point

Why do the sun and moon seem to move Why do the sun and moon seem to move each day?each day?

CalendarsCalendars The Egyptian The Egyptian

astronomers counted the astronomers counted the number of days between number of days between each first appearance of each first appearance of the star the star SiriusSirius in the in the morning sky.morning sky.

In this way, they found In this way, they found that there were about that there were about 365 365 days in each yeardays in each year. By . By dividing the year into 365 dividing the year into 365 days, the ancient days, the ancient Egyptians had created Egyptians had created one of the first calendars.one of the first calendars.

Earth’s OrbitEarth’s Orbit

Earth’s orbit around the sun actually takes Earth’s orbit around the sun actually takes slightly more than 365 days – about slightly more than 365 days – about 365 ¼ 365 ¼ days.days.

Four years of about 365 ¼ days each can Four years of about 365 ¼ days each can be approximated by taking 3 years of 365 be approximated by taking 3 years of 365 days and a fourth year of 366 days. You days and a fourth year of 366 days. You know this fourth year is a know this fourth year is a leap yearleap year..

““Moonth”Moonth”

Dividing the year into Dividing the year into smaller parts was difficult smaller parts was difficult also. Early people used also. Early people used moon cycles as a sort of moon cycles as a sort of calendar. The time calendar. The time between one full moon between one full moon and the next one is about and the next one is about 29.5 days. 29.5 days.

A year of 12 “moonths” A year of 12 “moonths” only adds up to 354 days. only adds up to 354 days.

““Moonth”Moonth”

The ancient Egyptians worked out a The ancient Egyptians worked out a calendar that had 12 months of 30 days calendar that had 12 months of 30 days each, with an extra 5 days that were not each, with an extra 5 days that were not part of any month.part of any month.

The Romans borrowed this calendar and The Romans borrowed this calendar and made changes to it. With more changes, it made changes to it. With more changes, it eventually became the calendar we know: eventually became the calendar we know: 11 months having 30 or 31 days, plus 11 months having 30 or 31 days, plus one month (February) having 28 or 29 one month (February) having 28 or 29 days.days.

Seasons on EarthSeasons on Earth

Most places outside Most places outside of the tropics have of the tropics have four distinct seasons: four distinct seasons: winter, spring, winter, spring, summer, and autumn summer, and autumn (fall). There are great (fall). There are great differences in differences in temperature from temperature from place to place.place to place.

Seasons on EarthSeasons on Earth Remember how sunlight hits Remember how sunlight hits

the Earth.the Earth. Earth’s tilted axis is the Earth’s tilted axis is the

cause of the seasons on this cause of the seasons on this planet.planet.

Latitude: a measurement of Latitude: a measurement of distance from the equator, distance from the equator, expressed in degrees north expressed in degrees north or south.or south.

The equator has latitude 0 The equator has latitude 0 degrees and the North Pole degrees and the North Pole has latitude 90 degrees has latitude 90 degrees north.north.

Equinox vs. SolsticeEquinox vs. Solstice Equinox: Means “equal Equinox: Means “equal

night.”night.” During an equinox, the length During an equinox, the length

of nighttime and daytime are of nighttime and daytime are about the same.about the same.

The The vernal equinoxvernal equinox, or spring , or spring equinox, occurs around March equinox, occurs around March 21, and marks the beginning of 21, and marks the beginning of spring in the Northern spring in the Northern Hemisphere.Hemisphere.

The The autumnal equinoxautumnal equinox occurs occurs around September 23. It around September 23. It makes the beginning of fall in makes the beginning of fall in the Northern Hemisphere.the Northern Hemisphere.

SolsticeSolstice

On two days each On two days each year, the noon sun is year, the noon sun is overhead at either overhead at either 23.5 degrees south or 23.5 degrees south or 23.5 degrees north. 23.5 degrees north. Each of these days is Each of these days is known as a solstice.known as a solstice. Summer Solstice: Summer Solstice:

June 21June 21 Winter Solstice: Winter Solstice:

December 21December 21

ReviewReview

Explain the process that Explain the process that causes day and causes day and night.night.

What two factors cause the What two factors cause the cycle of cycle of seasonsseasons??

Compare Compare rotationrotation and and revolutionrevolution..What do the words What do the words solsticesolstice and and equinoxequinox

mean? How are they related to the mean? How are they related to the position of position of Earth’s axisEarth’s axis..