Place and Time

1
Place and Time

• Seasons
• Origins in axial tilt
• Starting Dates

• Place
• Latitude and Longitude
• Antipodal Points

• Time
• Time Zones
• International Date Line

• Calendar
• Need for leap years.
• Gregorian Standard

2
Latitude

• Latitude angle are measured from the equator.
  They are labeled either North or South, depending
  on the hemisphere in which they are located.
• Points with the same latitudes are called
  parallels.
• The equator is at 0o.
• The poles are at 90o (N S).

3
Longitude

• Longitude angles are measured east or west from
  the prime meridian.
• Meridians are half-circles marking points of
  equal longitude.
• The prime meridian is 0o.
• The 180o meridian is the dateline.

4
Antipodes

• Antipodal points are points on Earth that are
  directly opposite one another.
• They are as far apart as it is possible to get.

Antipodes
Coordinate relations
5
Time Zones

• Time zones are defined so that noon (1200 p.m.)
  falls about the middle of the local daylight
  period.
• By convention, zones are set up with a one hour
  difference between each zone.

6
Theoretical Time Zones

• There are 24 time zones. In theory, each is 15o
  of longitude wide.
• Their centers are on meridians that are multiples
  of 15.
• Actual zone boundaries vary somewhat from this
  standard to accommodate local geography.

7
Clock Settings

• Because the earth rotates from west to east
  clocks are set so that the time in any zone is
  one hour later than the zone to its west, and 1
  hour earlier than the zone to its east.

8
International Dateline

• There are two places on earth where the calendar
  date changes as you cross a line.
• One in the western boundary of the midnight time
  zone.
• The other is the international date line.

9
Location of the Dateline

• In theory, the dateline is the 180o meridian.
• The actual location varies somewhat to
  accommodate local geography.

10
Calendar Adjustment

• The calendar date west of the Dateline is 1 day
  later than it is east of the line.
• Calendars are therefore set forward one day when
  the dateline is crossed moving west, and back one
  day when moving east.

11
Latitude and Daylight

• Sunlight strikes the earth more obliquely at high
  latitudes than at lower ones.
• High latitudes therefore receive less solar
  energy per square mile.

12
Axial Tilt

• The Earth's rotation axis is tilted 23.5o from
  the perpendicular to the plane of its orbit.
• This causes the angle between the surface and
  incoming sunlight to vary as the Earth moves
  along its orbit.

13
Key Dates

• Solstices are the points in the orbit where the
  rotation axis reaches maximum tip relative to the
  sun. These occur around June 21 or 22 and
  December 22 or 23.
• Equinoxes occur midway between the solstice. At
  these points, neither hemisphere is tipped toward
  the sun. These occur around March 20 or 21 and
  September 22 or 23.

14
June Solstice

• When the Northern Hemisphere is tipped toward the
  sun, sunlight strikes locations in the northern
  hemisphere less obliquely than if the Earth's
  rotation axis were not tipped.
• The situtation is reversed in the Southern
  Hemisphere.

15
December Solstice

• When the Northern Hemisphere is tipped away,
  sunlight strikes points there more obliquely than
  if there were no axial tilt.
• Again, the situation is reversed in the southern
  hemisphere.

16
Starting Dates

• Northern Hemisphere
• Spring-March Equinox
• Summer-June Solstice
• Fall September Equinox
• Winter December Solstice

• Southern Hemisphere
• Fall March Equinox
• Winter June Solstice
• Spring September Equinox
• Summer December Solstice

17
Effect on Climate

• The total amount of sunlight received at any
  latitutde during the year is essentially
  unchanged by this effect.
• More sunlight is received when a hemisphere is
  tipped toward the sun than when it is tipped
  away.
• The difference between summer and winter is
  determined by the tilt angle. A larger angle
  would produce more variation, a smaller angle,
  less.

18
Daylight Effects

• The length of the day also varies because of
  Earth's axial tilt.
• Daylight periods are longer dark periods in the
  hemisphere that is tipped toward the sun.
• Daylight periods are shorter than dark periods in
  the hemisphere that is tipped away.
• The effect becomes more pronounced with
  increasing latitude.

19
Artic Zones

• Points whose latitudes exceed 66.5o (N or S)
  experience at least 24 hrs of daylight at the
  summer solstice and 24 hrs of darkness at the
  winter solstice.
• These periods increase with increasing latitude.
• At the poles, the periods of daylight and
  darkness last 6 months. The sun rises at the
  time of the spring equinox and sets at the fall
  equinox.

20
Tropical Zones

• Points between 23.5o N and S latitudes will have
  the sun directly overhead at noon 1 or 2 days
  each year.

21
The Year

• The period of Earth's orbit around the sun is
  365.2422 days.
• A calendar based on a 365 day year will not
  remain synchronized with key events (like the
  equinoxes and solstices).
• Since a 365 day year is shorter than the actual
  period, occasionally years must be made longer to
  compensate. These are called leap years.

22
Calendar Standards

• Julian Calendar
• Leap years occur every 4 years.
• A year is a leap year if it is divisible by 4.

• Gregorian Calendar
• Years other than the last year of a century are
  leap years if divisible by 4.
• Century-ending years (1600, 1700, 2000, etc.) are
  only leap years if they are divisible by 400.

23
Comparison

• The Gregorian calendar has 3 fewer leap years in
  a 400 year period than the Julian.
• The Gregorian rule is a better match to the
  actual period of the Earth's orbit.
• The Gregorian calendar is the current world
  standard for dating civil events.