Axis
of the Earth
An
imaginary straight line running through the center of the Earth
from the North Pole to the South Pole. The earth is said to rotate
(spin) on this axis, which is "tilted" in relation to
the sun. This causes our seasons. |
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Day Length
Also
known as photoperiod. The length of time between
sunrise and sunset. |
Equinox
The
word means "equal night." It refers to the two moments
each year when the sun appears to cross the equator. The spring
(vernal) equinox is around March 21 and the fall (autumnal) equinox
is around September 23. Almost all places on Earth have about 12 hours of
daylight and 12 hours of night on the equinoxes. The sun doesn't really move, but it looks like it does
to us. That's because our Earth is tilted as it revolves around
the sun once a year. |
Spring
or fall equinox |
Greenwich
Mean Time (GMT)
or Universal Time (UT)
An international
time-keeping standard based on the local time in Greenwich, England
(0 degrees longitude). GMT or UT enables us to refer to things that
happen at the same moment in different time zones. (It would be
confusing to use "local time.") At any moment, GMT/UT is
the same everywhere on Earth. (That's why it's called universal
time.) What time is it now? Check the Greenwich
Mean Time clock.
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This map shows how many hours you need to add to or subtract from
your local time to get the GMT. (Click
to enlarge.) |
Hemisphere
A
mapping division meaning half of the Earth (sphere). First, picture
cutting the earth in half at the equator (0 degrees latitude). This
divides the Earth into the Northern Hemisphere (north of the equator)
and the Southern Hemisphere (south of the equator).
These are labeled in yellow on the illustration. Next picture cutting the earth in half at the Prime Meridian (0
degrees longitude). This divides the Earth into the Western Hemisphere
(west of the Prime Meridian) and the Eastern Hemisphere (east of
the Prime Meridian). These are labeled in red on the illustration. |
Click
to enlarge this image of the earth's hemispheres. |
Latitude
Imaginary horizontal mapping lines on the Earth.
They are known as "parallels" of latitude because they
run parallel to the Equator. The number of degrees of latitude shows
how far north or south of the Equator a specific location is.
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Longitude
Imaginary vertical mapping lines on Earth known
as "meridians" of longitude. The number of degrees of
longitude shows how far east or west of the Prime
Meridian a specific location is.
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Meridian
An imaginary line that runs vertically, north and south, from the
North Pole to the South Pole. All points on a meridian have the
same longitude. |
Photoperiod
Also called day length. The length of
time between sunrise and sunset. |
Prime
Meridian:
The Prime Meridian serves as the starting point
for longitude measurement, so is indicated as 0 degrees longitude.
The Prime Meridian passes directly over the British Royal Observatory
in Greenwich, England. (See illustration, above.) |
Revolution
of the Earth
The yearly 365 1/4 days trip the Earth takes
around the sun. We experience different seasons as the tilted Earth
revolves around the sun. |
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Rotation
of the Earth
The spinning or turning of the earth on its
axis. The Earth makes one complete rotation every twenty-four hours. |
We
see sunrise and sunset each day because the earth rotates.
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Solstice
The word means "sun
stop." It refers to the two moments each year when the sun
appears to be farthest from the equator. On the winter solstice
in our hemisphere (around December 22), the sun seems to reach its
most southerly point. It is our shortest day of the year. From there,
it seems to head north until it reaches its most northerly point.
That is our summer solstice (around June 21): the longest day of
the year. When the Northern Hemisphere has its summer solstice,
the Southern Hemisphere has its winter solstice, and vice versa.The sun doesn't really move, but it looks like it does
to us. That's because our Earth is tilted as it revolves around
the sun once a year. |
Winter
Solstice
in the Northern Hemisphere
Summer Solstice
in the Northern Hemisphere |
Twenty-Four
Hour Clock
Instead of keeping time on the basis of 12
a.m. hours and 12 p.m. hours, the 24-hour clock runs straight through.
It begins at 00:00 (midnight) and runs to 23:59 (11:59 PM).
This
is also known as "military time." |
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