Early Models of Solar System & Universe

Geocentric model

Earth-centered model with celestial bodies revolving around it

Heliocentric model

Sun-centered model with planets revolving around it

Nicolaus Copernicus

Mathematically proved the existence of the heliocentric model

Tycho Brahe

Provided additional observations supporting the heliocentric model

Johannes Kepler

Determined that planets move in elliptical paths

Kepler's First Law

Planets move in elliptical paths with the sun at one focus

Kepler's Second Law

A line joining a planet to the sun sweeps out equal areas in equal periods of time

Kepler's Third Law

The square of a planet's period is proportional to the cube of its semi-major axis

Galileo Galilei

Contributed to the heliocentric theory and formulated Newton's first law of motion

Terrestrial planets

Mercury, Venus, Earth, and Mars

Jovian planets

Jupiter, Saturn, Uranus, and Neptune

Astronomical Unit

Average distance between the sun and the earth

Celestial Sphere

Imaginary sphere of astronomical objects centered on the observer

Celestial Equator

Line on the celestial sphere directly above the Earth's equator

Celestial Pole

Point on the celestial sphere directly above a pole of the earth

Constellations

Patterns of stars named after objects, animals, or people

Ecliptic

Apparent path of the Sun on the celestial sphere

Degree

Unit of measurement on the celestial sphere, based on 360 degrees

Minute of arc

One-sixtieth of a degree of arc

What is a second of arc?

One-sixtieth of a minute of arc.

How did Eratosthenes measure the size of the Earth?

By comparing the Sun's direction off the vertical at two locations at the same time.

What was the difference in degrees between the Sun's direction at Syene and Alexandria?

7.2 degrees.

What fraction of a full circle is the 7.2 degrees difference?

1/50.

What was the radius of the Earth calculated by Eratosthenes?

6,600 km.

What is the current value of the Earth's radius?

6,378 km.

How is latitude defined?

Angular measurement in degrees north and south of the equator.

How is longitude defined?

Angular measurement in degrees east or west of the prime meridian (0-180 degrees).

What is a sidereal day?

Elapsed time between two successive crossings of the same meridian by a star other than the sun.

What is the International Date Line?

The line where the date changes when crossing it.

What happens when crossing the International Date Line westward?

The date is advanced into the next day.

What happens when crossing the International Date Line eastward?

One day is subtracted from the present date.

What is a Tropical year?

The time interval from one vernal equinox to the next vernal equinox.

What is a Sidereal year?

The time interval for Earth to make one complete revolution around the sun with respect to any particular star other than the sun.

When does the Summer solstice occur?

Around June 20 or 21, when the sun is at its most northern point from the equator.

When does the Winter solstice occur?

Around December 21 or 22, when the sun is at its most southern point from the equator.

What happens during the Vernal equinox?

The sun's position is directly over the equator, resulting in 12 hours of light and darkness around the world. It occurs around March 20 or 21.

What happens during the Autumnal equinox?

The sun's position is directly over the equator, resulting in 12 hours of light and darkness around the world. It occurs around September 22 or 23.