Physical Science 101

Revolution of the Earth

The Earth revolves around the Sun
Time it takes the Earth to complete one orbit around the Sun = 1 year
1 sidereal year = 365.25636 days
1 tropical year =
The orbit is an elliptical path
The orbit defines a plane - Ecliptic Plane
Average orbital velocity = 29.8 km/s
Orbital length = 940,416,480 km
Closest approach to the Sun = 147 million km
Farthest distance from the Sun = 152 million km

Rotation and Tilt of the Earth

The Earth rotates on its axis
One complete rotation = one day
1 day = 24 hours
Equatorial mean rotational velocity = 0.4651 km/s
Axis is tilted relative to the ecliptic at 23.5 degrees (23°27') from perpendicular to the ecliptic
The axial tilt varies between 22°-24°

Precession of the Axis

At present the Earth's axis appears to point towards Polaris (North Star)
Over time the axis moves - precesses
The axis completes one complete precession in 25,800 years


Seasons are caused by variations in the amount of solar radiation received
Controlled by two main factors:
Seasons begin and end at an Equinox and Solstice


The Circle of illumination passes through the Earth's axis; each hemisphere is equally illuminated
Subsolar Point is located at the Equator
Subsolar Point is the point on the Earth's surface where the sun's rays are perpendicular to the surface
Vernal Equinox - March 21
Autumnal Equinox - September 23


Circle of illumination is tangent to the Arctic or Antarctic Circle
One hemisphere receives more sunlight than the other
Winter Solstice - December 22
Subsolar point is located on the Tropic of Capricorn
Summer Solstice - June 21
Subsolar point is located on the Tropic of Cancer

Tropic and Arctic Circles

Tropics of Cancer and Capricorn are located at 23.5 degrees N and S Latitude
Marked by the subsolar point on the Solstices
Arctic and Antarctic Circles are located at 66.5 degrees N and S Latitude
Marked by the circle of illumination on the Solstices


What is time?

Global Time vs. Geologic Time
Units of timekeeping?

Global Time

Solar Time Keeping
Apparent Solar Time: Based on position of Sun in the sky
Apparent Solar Day: The time it takes the sun to go from high noon to high noon
Mean Solar Day: Sun moves at constant rate of 24 hours per day

Lunar Time Keeping: based on movement or phases of Moon
Sidereal Month:
Synodic Month:

Time Zones

Based on Mean Solar Time
High noon at the Standard Meridian Each
Time zone are 15 degrees of Longitude for a total of 24 time zones

Geologic Time

What is Geologic time?
1 year is the time it takes the Earth to complete one orbit around the Sun

Two main methods of looking at geologic time
Relative Time - Absolute Time