earth science 22.2a earth-sun system earth sun system

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Earth Science 22.2A Earth-Sun System Earth Sun System

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Earth Science 22.2A Earth-Sun System

Earth Sun System

Earth Science 22.2A Earth-Sun System

If you gaze away from city lights on a clear night, it will seem that the stars produce a spherical shell surrounding Earth.

This impression seems so real that it is easy to understand why so many early Greek regarded the stars as being fixed to a solid celestial sphere.

The sun and moon however constantly change their positions against the stars.

Prehistoric people built observatories to chart the motions.

Structures such as Stonehenge were attempts to make better predictions of the sun’s cyclic movement in the sky.

Earth Science 22.2A Earth-Sun System

At the beginning of the summer in the northern hemisphere (summer solstice on June 21 or 22), the rising sun comes up directly behind the heel stone of Stonehenge.

Besides keeping this calendar, Stonehenge may also have provided a method of determining eclipses.

In today’s lesson, we will learn more about the movements of bodies in space that cause events such as eclipses.

Rotation vs RevolutionMotions of the Earth:

The two main motions of the Earth are rotation and revolution.

Rotation is the turning, or spinning, of a body on it’s axis. Revolution is the motion of a body, such as a planet or moon, along it’s orbit around some point in space. For example, Earth revolves around the sun and the moon revolves around the Earth.

Earth also has another very slow movement known as precession, which is a slight movement over a period of 26,000 years of the Earth’s axis.

Rotation

Revolution

Rotation vs Revolution

Rotation:

The main results of Earth’s rotation are day and night.

Earth’s rotation has become a standard method of measuring time because it is so dependable and easy to use.

Each rotation of the Earth takes about 24 hours.

Rotation

Apparent Solar Day

You may be surprised to learn that we can measure Earth’s day in two different ways.

Most familiar is the apparent solar day, from one noon to the next, which on average is about 24 hours.

Noon is when the sun has reached it’s highest point in the sky.

Rotation

Sidereal Time

The sidereal day, on the other hand, is the time it takes Earth to complete one complete rotation (360 degrees) in relation to a star other than our sun.

The sidereal time is measured by the time it takes for a star to reappear at the identical position in the sky where it was observed the day before.

The sidereal day has a period of 23 hours, 56 minutes, and 4 seconds which is almost 4 minutes shorter than the average solar day.

Sidereal Time

The difference between the mean solar day and the sidereal day is due to the fact that the direction to far off stars changes very little day to day while the direction to the sun changes by almost 1 degree per day (another 4 minutes of rotation) due to the Earth’s orbit around the sun.

The diagram at right shows how each day has a 4 minute difference between the sidereal time and the average solar day length.

Sidereal Time

Why do we use the mean solar day instead of the sidereal day as a measurement of our day than?

In sidereal time, noon occurs 4 minutes earlier each day.

Therefore, after 6 months, noon would be occurring at midnight.

Astronomers however use sidereal time because the stars appear in the same position in the sky every 24 sidereal hours.

Sidereal Time

Revolution:

Earth revolves around the sun in an elliptical orbit at an average speed of 107,000 kilometers per hour.

Its average distance from the sun is 150 million kilometers.

But because earth’s orbit is an ellipse, Earth’s distance from the sun changes.

Sidereal Time

At perihelion, Earth is closest to the sun, about 147 million kilometers away.

Perihelion occurs about January 3rd each year.

At aphelion, Earth is farthest away from the sun, about 152 million kilometers away.

Aphelion occurs about July 4th.

So Earth is farthest from the sun in July and closest in January.

Ecliptic

Because of the Earth’s annual movement around the sun, each day the sun appears to move among the constellations.

The apparent annual path of the sun against the backdrop of the celestial sphere is called the ecliptic.

Generally, the planets and the moon travel in nearly the plane as the Earth.

So their paths on the celestial sphere lie near the ecliptic.

Earth’s Axis and seasons The imaginary plane that connects

Earth’s orbit with the celestial sphere is called the plane of the ecliptic.

The projection of Earth’s equator onto the sky is the celestial equator.

Earth’s axis of rotation is titled about 23.5 degrees toward the plane of the ecliptic.

Because of the Earth’s tilt, the apparent path of the sun and the celestial equator intersect each other at an angle of 23.5 degrees.

Because of this inclination of the Earth’s axis to the plane of the ecliptic, Earth has it’s yearly cycle of seasons.

Earth’s Axis and seasons

When the apparent position of the sun is plotted on the celestial sphere over a period of a year’s time, it’s path intersects the celestial equator at two points.

From a Northern Hemisphere point of view, these intersections are called the spring equinox (March 20-21) and the autumnal equinox (September 22-23).

Earth’s Axis and seasons

On June 21st or 22nd, the date of the summer solstice, the sun appears 23.5 degrees north of the celestial equator.

Six months later, on December 22nd -23rd, the date of the winter equinox, the sun appears 23.5 degrees south of the celestial equator.

Precession

Precession:

The slow movement of Earth, called precession, is the motion of Earth’s axis as it traces out a circle in the sky.

Earth’s axis varies in tilt between 21.5 degrees and 24.5 degrees within a cycle of 41,000 years.

This cycle is very important in affecting climate change as we have noted in previous chapters.

Precession In addition, the direction that

the axis points continually changes. This movement is similar to the wobble of a spinning top.

At the present time, the axis points toward the bright star Polaris (the current North Star or pole star).

In about 13,000 years, the direction will shift to point toward the star Vega and this will than become the pole star.

The period of precession, the amount of time for the axis to complete one cycle, is 26,000 years.

26,000 year cycle of the precession

Earth-Sun motion:

In addition to it’s own movements, Earth accompanies the sun as the entire solar system speeds in the direction of the bright star Vega at 20 kilometers per second.

Also, the sun, like other nearby stars, revolves around the Milky Way Galaxy.

This trip takes 226 million years to traverse at speeds approaching 250 kilometers per second.

Earth-Sun motion:

The galaxies themselves are also in motion.

The Milky Way Galaxy is presently approaching one of it’s nearest galactic neighbors, the Andromeda Galaxy.

The motions of Earth are many and complex and it’s speed through space is very great.

Earth-Sun motion: