Physical Geography 101
Time


Seasons

Seasons are caused by variations in the amount of solar radiation received
Controlled by two main factors:
1.
2.
Seasons begin and end at an Equinox and Solstice

Equinox

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

Solstice

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

Earth's Vital Statistics

Radius 6,371 km
Circumference 40,000 km
Mass 6x1024 kg
Density 5.519 g/cc
Surface Area 510,050,100 km2
Surface Area - land 29.2 %
Surface Area - water 70.8 %
Volume 1,083,157,900,000 km3

Time

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: The time it takes the Moon to complete one full orbit measured with respect to the stars
Synodic Month: The time it takes the Moon to complete one cycle of phases new moon to new moon full moon to full moon measured with respect to the Sun

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

Relative Time

Measure of the sequence of events without knowing the exact date at which each event occurred
What happened first?
What happened next?

Click here for additional information on Relative Time.

Absolute Time

Measure of the actual date at which an event occurred
Mt. St. Helens erupted on May 18, 1980
Observation of an event - observer records the date of an event - works well for recent events, historic events

Radiometric Dating

Based on the fact that certain elements are unstable and decay, releasing radiation
Isotopes decay at a constant rate
This can be used to determine an age for various objects

Click here for additional information on Radiometric Dating.

Geologic Time Scale

Arbitrary chronologic arrangement or sequence of geologic events that are divided into Eons, Eras, Periods, and Epochs
Contains information based on both relative and absolute dating techniques

Click here for additional information on the Geologic Time Scale.