Earth in Space 19-1

Astronomy is the study of objects beyond the Earth’s atmosphere.

It is one of the oldest sciences, as ancient people studied the heavens for agricultural and religious reasons.

Rotation Ancient astronomers thought

that the sun, moon, and stars were moving around the Earth.

This is because of the Earth’s rotation.

The Earth spins on its axis, an imaginary line that passes through the center of the planet and both poles.

Rotation at Night

Axis

The Earth spins on its axis at about 1,600 km/hour at the equator towards the east.

This is why the sun rises in the east and sets in the west.

The axis is currently pointing toward the star Polaris, the North Star.

Is the north star always Polaris?

A slow wobble in the Earth's daily rotation causes the Earth's north pole to trace a circle among the stars every 26,000 years.

Because of this motion, called precession, the star nearest the pole is not always the same.

Architects of the great Egyptian pyramid used Thuban (TOO-bahn), a star in the constellation Draco the Dragon, for their north star.

Polaris, the tip of the Little Dipper's handle, currently lies within three-quarters of a degree from the polar point, and will reach its closest proximity--under half a degree--in the year 2102 AD.

No matter where you are in the northern hemisphere, when you face Polaris you are facing north. Polaris now points northward more accurately than a magnetic compass.

In 14,000 years Vega will become the pole star.

This extremely slow but steady cycle of precession was discovered around 150 B.C by the ancient astronomer Hipparchos.

Revolution Revolution is the

movement of one object around another caused by gravity.

One complete revolution is called a year.

The path around the object is called an orbit.

The Earths orbit is a slightly flattened circle called an ellipse. It travels at 30-km/sec.

It takes the Earth about 365.25 days to make one orbit of our Sun.

Day and Night

Rotation causes the Earth to have day and night.

It takes about 24 hours for the Earth to rotate on its axis once. We call this 24 hour cycle a day.

Tilt The Earth’s axis is

tilted 23.5° from the vertical.

The tilt is always towards the North Star, so part of the year the north axis is pointed towards the sun, and part of the year it is pointed away.

This causes the Earth to have seasons.

Earth’s Seasons

The Equator is warmer because it receives sunlight more directly than the poles.

Without the tilt Earth would not have seasons.

Summer and winter are not affected by changes in Earth's distance from the sun. In fact, when the Northern Hemisphere is having summer, Earth is at its greatest distance from the sun.

Calendars

The Earth’s year is 365.25 days. That is why our calendar has a leap year every four years.

Calendars were originally made to make astronomical predictions.

Lunar Calendars

The moon revolves around the Earth every 27.3 days.

This is why people that use lunar calendars will have years with 12 or 13 months.

Solstices Sun is directly at 23.5° north

or south latitude. Summer (June 21) solstice

has the longest daylight/shortest night in the Northern Hemisphere.

The winter solstice (December 21) is the shortest daylight/longest night in the Northern Hemisphere.

Of course these are reversed if you live in Australia!

Equinoxes Means “equal night.”

Days and nights are equal in time.

Axis is not tilted towards or away from the Sun.

Sun is directly over the equator.

Vernal equinox (spring) occurs around March 21st.

Autumnal equinox occurs around September 23rd.

Again if you lived in the Southern Hemisphere these would be reversed!

Tropic of Cancer Northern most point where the sun can be

directly overhead at noon. Located at 23.5 degrees north latitude (pretty

much the same at the tilt of the Earth’s axis). Occurs on the summer solstice.

Tropic of Capricorn

Southern most point where the sun can be directly overhead at noon.

Located at 23.5 degrees south latitude (pretty much the same at the tilt of the Earth’s axis).

Occurs on the winter solstice.

Arctic/Antarctic Circle

Point where at least one day a year the sun never sets/rises.