how do we measure LONGTITUDE? in clear and simply method pls~?

Question

i know we use the distance from the star and the angle to read a latitude, but what about longtitude? i'm still abit fuzzy here

Answer 1

You look at the sun an determine local noon. Then you look at your watch, set for GMT.

If there are 8 hours and 36 minutes difference, you are a little over one third of the world away from Greenwich, England. (24 hours is one lap.)

Answer 2

Longitude cannot be directly measured, except from space or with GPS. Traditionally, one determined longitude by keeping a clock set to Greenwich time and used a sextant to sight high noon at one's current location. The time difference would then be used to calculate the longitude of one's location.

However, this required a clock that could keep time to within a few seconds accuracy over a voyage duration of several months. This was actually not even possible until 1761, when William Harrison demonstrated a naval clock that could keep time well enough to be used for determination of longitude and position within 10 miles:
http://en.wikipedia.org/wiki/John_Harrison

Answer 3

A geographic coordinate system expresses every location on Earth by two of the three coordinates of a spherical coordinate system which is aligned with the spin axis of Earth. Borrowing from theories of ancient Babylonians, later expanded by the famous Greek thinker and geographer Ptolemy, a full circle is assigned 360 degrees (360°).

latitude (Lat.) is the angle between any point and the equator. Lines of constant latitude are called parallels. They trace circles on the surface of Earth, but the only parallel that is a great circle is the equator (latitude=0 degrees), with each pole being 90 degrees (north pole 90° N; south pole 90° S).
longitude (Long.) is the angle east or west of an arbitrary point on Earth: The Royal Observatory, Greenwich (UK) is the international zero-longitude point (longitude=0 degrees). The anti-meridian of Greenwich is both 180°W and 180°E. Lines of constant longitude are called meridians. The meridian passing through Greenwich is the Prime Meridian. Unlike parallels, all meridians are halves of great circles, and meridians are not parallel: they intersect at the north and south poles.
By combining these two angles, the horizontal position of any location on Earth can be specified.

For example, Baltimore, Maryland (in the USA) has a latitude of 39.3° North, and a longitude of 76.6° West (39.3° N 76.6° W). So, a vector drawn from the center of Earth to a point 39.3° north of the equator and 76.6° west of Greenwich will pass through Baltimore.

This latitude/longitude "webbing" is known as the common graticule. There is also a complementary transverse graticule (meaning the graticule is shifted 90°, so that the poles are on the horizontal equator), upon which all spherical trigonometry is ultimately based.

Traditionally, degrees have been divided into minutes ( ′ ) and seconds ( ″ ). There are several formats for degrees, all of them appearing in a Lat.-Long. order:

DM Degree:Minute (49:30.0-123:30.0)
DMS Degree:Minute:Second (49:30:00-123:30:00)
DD Decimal Degree (49.5000-123.5000), generally with 4 decimal numbers.
To change from DM or DMS to DD, Decimal degrees = whole number of degrees, plus minutes divided by 60, plus seconds divided by 3600. Decimal division is now the most common and standard.

The equator is obviously an important part of this coordinate system, it represents the zeropoint of the latitude angle, and the halfway point between the poles. The equator is the fundamental plane of the geographic coordinate system. All spherical coordinate systems define such a fundamental plane.

Latitude and Longitude values are established based on an associated Geodetic system or datum such as WGS 84. In other words, the same exact point on the earth’s surface will be expressed by different latitude and longitude values, depending on the reference datum.

In popular GIS software, data projected in latitude/longitude is often specified via a 'Geographic Coordinate System'. For example, data in latitude/longitude with the datum as the North American Datum of 1983 is denoted by 'GCS_North_American_1983'.

Answer 4

If youuse a sextant to measure the positions of at least three stars and you've got an accurate clock and book of tables you can work out both your latitude and longitude. Alternatively if you've got an accurate clock you can work out your longitude by observing the time of sunrise or sunset. These days of course, there are easier methods of navigation; GPS, GLONASS and soon Galileo, which use satellites, DME which tells you your distance from a radio beacon, VOR which tells you your bearing from a radio beacon, LORAN which tells the difference between your distances from two or more radio beacons, military variations of these, like TACAN, and inertial navigationwhich continuously integrates your acceleration twice to work out your distance travelled from where you took off.

Answer 5

15 degrees for every 400kilometers