Earth’s Orbit Around the Sun

Objective 1.1.1: Explain Earth’s motion through space, including

precession, nutation, the barycenter, and its path about the galaxy.

Essential Standard 1.1: Explain Earth’s role as a body in space.

Earth’s Orbit Around the SunThe Earth orbits the Sun, every 365.25 days, or

each year.

To account for the 0.25, one day is added to the calendar every four years. We call that day a leap day

and the year it occurs a leap year.

Note

Check

Distance to the SunBecause Earth’s orbit is an ellipse with the Sun at one focus, the Earth is at different distances from the Sun

at different times of the year.

PerihelionThe Earth is closer to the Sun in January.

AphelionThe Earth is farther from the Sun in July.

The average distance to the Sun, throughout the year, is 93 million miles.

Center of MassWhile we say the Earth orbits the Sun, that is not

whole truth.

The Earth and the other planets actually orbit around the center mass between each individual

planet and the Sun.

What is a Center of Mass?Every object has a center of mass, sometimes called the center of gravity, at the point at which it can be

balanced.

Sometimes the center of mass or gravity is located in the center of the object.

What is a Center of Mass?But sometimes the center of mass is not in the

center. Some parts of the object may have more mass than other parts of the object.

The sledge hammer has more mass on one end, so the center of mass is closer to the

heavier end.

BarycenterIn space, two or more objects orbiting each other also have a center of mass. It is the point at which the objects actually orbit. This point is called the

barycenter of the objects.

The barycenter is usually closer to the object with the most mass.

Barycenter of Earth & SunSince the Sun has so much mass, compared the

Earth. The Sun is like the head of the sledgehammer.

The barycenter between the Earth and the Sun is very close to the center of the Sun.

Barycenter

Barycenter of Jupiter & SunJupiter is a lot larger than Earth with a mass 318

times greater than Earth’s mass.

As a result, the barycenter of Jupiter and the Sun is actually just outside the Sun’s surface.

Barycenter of the Solar SystemActually, all the planets combined create a barycenter

which they and the Sun orbit around.

As planets change their locations

within their own orbits, the

barycenter of the solar system shifts.

As a result of the shifting solar system barycenter, the Sun appears to wobble as it completes a revolution.

Click on image to see animation of

the Sun revolving around the Solar

System’s barycenter.

Earth’s RotationAs the Earth orbits the Sun, the Earth rotates around

its own axis, every 24 hours or one day.

Day

Night

The side of Earth facing the Sun, receives light, and experiences day time.

The side of Earth facing away from the Sun, doesn’t

receive light, and experiences night time.

Click on link for video of

Night time – Day time.

Counter-Clockwise RotationAs the Earth rotates around its own axis, it does so in

a counter-clockwise direction, causing the Sun to rise in the East and set in the West, when viewed

from Earth.

Imagine looking down

from the North Pole, in

the above image. Then

look at the image on

the right.

Earth’s ShapeEarth’s gravity pulls equally on all the materials in

and on Earth towards the center.

This pull towards the center, causes Earth to form a round sphere shape.

However, because Earth rotates, the Earth is not a perfectly round sphere.

Equatorial BulgeMuch like the skirt of an ice skater, the material at the equator wants to spin outward, as Earth rotates. This

creates what is called an equatorial bulge.

Exaggerated

Equatorial Bulge

Think of the water in

the cup on a string.

The result of the equatorial bulge is that the diameter of Earth at the equator is about 50 km greater than

the diameter at the poles.

Equatorial Bulge

Earth’s Ecliptic PlanetIf you were to draw an imaginary line from the Earth to the Sun, during any part of Earth’s orbit about the Sun, you would find that the Earth remains on one

plane in space, called the Ecliptic Plane.

Solar Systems Ecliptic PlaneAll of the planets actually orbit on or very near the

same plane as Earth’s ecliptic plane, with some slight variations.

Earth’s Tilted AxisThe axis about which Earth rotates about its own axis, it is tilted at a 23.5 o angle to the

plane of the ecliptic.

Equatorial PlaneIf you were to draw an imaginary line outwards from Earth’s equator, into space, you would create Earth’s

equatorial plane.

Because the Earth’s axis is tilted, Earth’s

equatorial plane and Earth’s ecliptic plane are

not lined up or on the same plane.

PrecessionMuch like a spinning top will wobble before it falls

down, the tug on Earth’s equatorial bulge by the Sun and the Moon, trying to line it up with the plane of the ecliptic, causes the Earth to wobble as it rotates

around its axis.

This wobble is called Precession.

Changing View of the StarsIf you were to draw an imaginary line from Earth’s rotational axis out into space, overtime, that line would trace out an arc shape due to precession.

Precession does not change the tilt of Earth’s axis but it

does change our view of the stars.

North Star

Back when the Egyptians built the pyramids, Thuban

was the north star.

Currently, Polaris is the North Star.

North Star

In about 7,000 years, Vega will be

the north star.

It takes Earth 26,000 years to complete one precessional cycle. In other words, once Polaris is no longer our north star, it will take 26,000 years for it

to once again be the north star.

NutationAs the Earth completes the slow precessional cycle, the Moon tugs on Earth’s equatorial bulge, causing the tilt of Earth’s rotational axis to change half of a

degree in either direction. (nodding motion)

This change in the tilt of Earth’s

rotational axis is called nutation.

Change in SeasonsBecause the tilt of Earth’s axis is related to Earth’s

seasons, nutation does affect Earth’s season, creating harsher winters and hotter summers.

It takes Earth 18.6 years to complete one

nutational cycle.

The EndSpace Cadet