Time & Zones (Latitude and Longitude)

Time & Zones (Latitude and Longitude)

Latitude and Longitude

Any location on Earth is described by two numbers- its latitude and its longitude.

Latitude

On a globe of the Earth, lines of latitude are circles of different size. The longest is the equator, whose latitude is zero, while at the poles-at latitudes 90° north and 90° south (or -90°) the circles shrink to a point.

Longitude

On the globe, lines of constant longitude ('meridians') extend from pole to pole.

Every meridian must cross the equator. Since the equator is a circle, we can divide it-like any circle-into 360 degrees, and the longitude of a point is then the marked value of that division where its meridian meets the equator.

For historical reasons, the longitude (meridian) passing the old Royal Astronomical Observatory in Greenwich, England, is the one chosen as zero longitude. Located at the eastern edge of London, the British capital, the observatory is now a public museum and a brass band stretching across its yard marks the 'prime meridian'.

A line of longitude is also called a meridian, derived from the Latin, from meri, a variation of 'medius' which denotes 'middle', and diem, meaning 'day'. The word once meant ""noon" , and times of the day before noon were known 'as 'ante meridian', while times after it were 'post meridian' Today's abbreviations a.m. and p.m. come from these terms, and the Sun at noon was said to be "passing meridian". All points on the same line of longitude experienced noon (and any other hour) at the same time and were therefore said to be on the same 'meridian line."

Local Time (LT) and Time Zones

• Two important concepts, related to latitude and (especially) longitude are Local Time (LT) and Universal Time (UT).
• Longitudes are measured from zero to 180° east and 180° west (or -180°), and both 180-degree longitudes share the same line, in the middle of the Pacific Ocean.
• As the Earth rotates around its axis, at any moment one line of longitude "the noon meridian" faces the Sun, and at that moment, it will be noon everywhere on it. After 24 hours the Earth has undergone a full rotation with respect to the Sun, and the same meridian again faces noon. Thus each hour the Earth rotates by 360/24 = 15 degrees.

The Date Line and Universal Time (UT)

• Longitude determines only the hour of the day-not the date, which is determined separately. The international date line has been established-most of it following the 180th meridian-where bycommonagreement, whenever we cross it the date advances one day (going west) or goes back one day (going east).
• That line passes the Bering Strait between Alaska and Siberia, which thus have different dates, but formost of its course it runs in mid-ocean and does not inconvenience any local time keeping.
• Astronomers, astronauts and people dealing with satellite data may need a time schedule which is the same everywhere, not tied to a locality or time zone. The Greenwich Mean Time, the astronomical time at Greenwich (averaged over the year) is generally used here. It is sometimes called Universal Time (UT).
• The Royal Observatory at Greenwich in London was adopted as the site of the Universal Time meridian of longitude by the International Meridian Conference—a meeting of 25 nations in Washington DC on 22^nd October 1884. The decision formalised the international standard time reference point of Greenwich Mean Time (GMT), which is still used to delineate world time zones.

Indian Standard Time (1ST)

This is the imaginary line passing through mid of the standard Meridian in the middle of the country and standard time of the country is followed all over the country to avoid the inconvenience caused by the difference in local times of different places in the country. 821/2°E or 82°30'E longitude, passing through Mirzapur (Amravati Chauraha) Uttar Pradesh, is the Standard Meridian of India. The time at this longitude is the standard time of India, called the Indian Standard Time (1ST).

Lines crossing different countries

Tropic of Cancer: Taiwan, China, Myanmar, Bangladesh, India, Oman, UAE, Saudi Arabia, Egypt, Libya, Niger, Algeria, Mali, Mauritania, South Sahara, Bahamas, Mexico

Tropic of Capricorn: Chile, Argentina, Paraguay, Brazil, Namibia, Botswana, South Africa, Mozambique, Madagascar, Australia, French Polynesia, Fiji, Tonga

Equator: Ecuador, Columbia, Brazil, Sao Tome & Principe, Gabon, Congo, Democratic Republic of Congo, Uganda, Kenya, Somalia, Maldives, Indonesia, Kiribati

Heat Zones of The Earth Torrid Zone, Frigid Zone

• This is also referred to as Tropical zone. The Tropics is a region on the Earth surrounding Equator by the Tropic of Cancer in the northern hemisphere at 23°26'16" N (approx.) and the Tropic of Capricorn in the southern hemisphere at 23°26’ 16" S (approx.). The Tropics include all the areas on the Earth where the sun reaches a point directly overhead at least once in a year.
• This area receives maximum heat and is called the Torrid (hot) Zone.

Frigid Zone

• Near the polar regions, the rays of the Sun are very slanting and so it is very cold.
• The region/area between the Arctic Circle and the North Pole in the Northern Hemisphere is called the Frigid zone.
• There are similar regions in the Southern Hemisphere between the Antarctic Circle and the South Pole, also called the Frigid Zone (frigid means cold).

Earth and its Rotation with the Sun

Rotation of the Earth

• The Earth spins (rotates), west to east on its axis once in 24 hours approximately.
• The Earth's axis is not vertical. It makes an angle of 23°30' with the vertical or 66°30' with the plane of the Earth's orbit.
• The Earth's axis always remains pointed in the same direction (towards the Pole Star) as the Earth moves around the Sun. The tilt of the Earth's axis is known as the inclination of the Earth's axis.
• Movements of tides are mostly determined by rotation of the Earth.

Sidereal day: The period of time during which a star describes a complete circle in its apparent journey around the pole star, representing the period of one rotation of the earth on its axis and equal to 23 hours 56 minutes 4 seconds. It is thus about 4 minutes shorter than the mean solar day.

Solar day: The average period taken by the earth in making one rotation on its axis in relation to the sun-24 hours.