i. Latitude and Longitude - Latitude is the angular disance of a point on the Earth’s surface, measured in degrees from the centre of the Earth. Longitude is the angular distance, measured in degrees along the equator, east or west of the Prime Meridian (the meridian that passes through Greenwich near London).
ii. International Date Line – It is the line that passes through 180º East or West meridian of longitude which falls on the opposite side of the Greenwich meridian. One day is added when this line is crossed East to West and a day is deducted when the line passes West to East.
iii. Greenwich Mean Time (GMT) – GMT refers to the 0º longitude that passes through Greenwich near London.
iv. Indian Standard Time (IST) - IST refers to the longitude that passes through Allahabad and is hours ahead of the GMT.
v. Solstice - It is the date on which the Sun shines vertically over a tropic when the days and nights are the longest and the shortest. On the 22nd of June the sun shines vertically over the Tropic of Cancer and this date is termed as Summer Solstice for the Northern Hemisphere and Winter Solstice for the Southern Hemisphere. Similarly, on December 22nd, the Sun shines vertically over the Tropic of Capricorn. Hence it is Summer Solstice for Southern Hemisphere and Winter Solstice for the Northern Hemisphere.
vi. Equinox - It means two days during the year when the day and night are almost equal throughout the World because the Sun shines vertically over the Equator. March 21st is called the Vernal Equinox and September 23rd is called the Autumnal Equinox.
vii. Equator - The Equator represents the imaginary line passing around the Earth midway between the North and South poles. It divides the Earth into two equal parts.
viii. Meridians - They represent the imaginary lines drawn out on the global map from pole to pole and perpendicular to the Equator. Prime Meridian is the 0 degree meridian, which passes through Greenwich, a place near London.
ix. Aphelion - Aphelion refers to that position of the Earth in its orbit, when it is at the greatest distance from the Sun (about 152 million kms). The Earth reaches this position during the northern summer. This term also refers to the position of any other planet or a comet, when it is at its greatest distance from the Sun.
x. Perihelion - Perihelion refers to the nearest point of a heavenly body in its orbit around the Sun. The Earth at perihelion is at a distance of about 147 million kms from the Sun.
xi. Apogee - Apogee refers to the position of the orbit of the moon when it is at its greatest distance from Earth. This is approximately 406,000 kms for the Earth.
xii. Perigee - Perigee refers to the point of the moon when it is at its minimum distance from the Earth. This distance is approximately 364,000 kms.
xiii. Cyclone - Any area of low pressure around which the air turns in the same direction as the Earth is a cyclone. In the Indian Ocean, a tropical storm with masses of air turning rapidly round a low pressure area is called a tropical cyclone.
xiv. Typhoon - It is the name given to a tropical cyclone in the Far East.
xv. Hurricane - It is a tropical storm in the Caribbean or West Pacific Ocean, with extremely strong winds. The wind force reaches 12 on the Beaufort scale.
xvi. Tornado - It is a type of whirlwind which is formed by rising air currents associated with large cumulonimbus clouds. It rotates in an anticlockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere.
xvii. Estuary - It is the tidal mouth of a river where the salt water of the tide meets the fresh water of the river current.
xviii. Isobars - Lines on the map denoting places experiencing equal pressure.
xix. Isobaths - Lines denoting equal depths in the Sea.
xx. Isobronts - Lines joining places experiencing a thunderstorm at the same time.
xxi. Isochrones - Lines joining places located at equal travel time from a common centre.
xxii. Isogonics - Lines joining places with the same magnetic declination.
xxiii. Isorymes - Lines denoting places experiencing equal frost.
xxiv. Isoseismics - Lines denoting places experiencing equal seismic activity.
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