Category Science

As the earth spins, different parts of it face the Sun. Therefore, it cannot be the same time all over the world at the same moment. When it is the middle of the night in one country, it is dawn in another part of the world. To keep expressions of time consistent in every part of the world, the Earth is divided into 24 time zones, each one exactly an hour apart.

Everyone on the planet wants the sun to be at its highest point in the sky (crossing the meridian) at noon. If there were just one time zone, this would be impossible because the Earth rotates 15 degrees every hour. The idea behind multiple time zones is to divide the world into 24 15-degree slices and set the clocks accordingly in each zone. All of the people in a given zone set their clocks the same way, and each zone is one hour different from the next.

In the continental United States there are four time zones Eastern, Central, Mountain and Pacific. When it is noon in the Eastern time zone, it is 11 a.m. in the Central time zone, 10 a.m. in the Mountain Time zone and 9 a.m. in the Pacific Time zone.

All time zones are measured from a starting point centered at England’s Greenwich Observatory. This point is known as the Greenwich Meridian or the Prime Meridian. Time at the Greenwich Meridian is known as Greenwich Mean Time (GMT) or Universal Time. The Eastern Time zone in the United States is designated as GMT minus five hours. When it is noon in the Eastern Time zone, it is 5 p.m. at the Greenwich Observatory. The International Date Line (IDL) is located on the opposite side of the planet from the Greenwich Observatory.

Why is the Greenwich Observatory such a big deal? A bunch of astronomers declared the Greenwich Observatory to be the prime meridian at an 1884 conference. What’s funny is that the observatory moved to Sussex in the 1950s, but the original site remains the prime meridian.

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The year is not an exact number of days but about 365 and one quarter days. By adding an extra day into the calendar every four years, we ensure that the year does not gradually become out of step with the seasons.

The solar year is not exactly 365 days. It is 365.24219 days. That means it is almost a quarter of a day longer than our standard 365 day calendar. Over time, that adds up. After four years, the calendar would be off by almost a full day. After 120 years, the calendar would be off by a month. Since one of the primary uses of a calendar is to help people know when to plant their crops, that ever increasing error is problematic. That’s why we have leap years – to correct for that error.

Julius Caesar, in 45 B.C., implemented the practice of adding a leap day to the calendar every four years. In the short term, this addressed the problem, but our story doesn’t end here.

The solar year is 365.24219 days long, not the 365.25 days that the leap year concept was built to address. The difference between those two numbers equates to about eleven minutes per year or a full day every 128 years. By the time of Pope Gregory in 1582, this error had built up considerably. Gregory reformatted the calendar, removing ten days from one year to get back in sync and then redefining the use of leap years to add the following rule: centennial years (e.g. 1800, 1900, and 2000) are not to be leap years unless they are evenly divisible by 400.

That correction is based upon a solar year length of 365.2425 days. That’s much better, but still slightly different than 365.24219 and that difference of 0.00031 days means that after about three thousand years, the Gregorian calendar will be off by a day.

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After the day, the next measurement of time that early people used was probably the month. They noticed that the Moon changed shape on a regular cycle, with 28 days passing from one full Moon to the next. In fact, the true figure is about 29.5 days. This is a lunar month. The phases of the Moon do not divide exactly into the 365 or 366 days in a year, so over time the months that we use today, known as calendar months, came to have slightly different lengths. April, June, September and November have 30 days each. All the other months have 31 days, except February, which has 28 days, or 29 in a leap year.

The Gregorian calendar is a solar calendar; this sort of calendar gives a date based on the position of the sun in relation to the stars behind it. The seasons are based on the equinoxes and correspond to the declination of the sun.

A lunar calendar is different. The cycles it records are based on the monthly phases of the moon. A month starts when the moon reaches a certain phase in a certain location and a year is defined as 12 months of this kind. The months in the Gregorian calendar are not calculated in this way.

First off, the lunar year is only 354 days, give or takes a few. This means holidays and festivals determined by a lunar calendar, such as Easter, the Chinese New Year, or Rosh Hashanah, will seem to shift by as much as 11 days a year with respect to a solar calendar. Also, since months are determined by the phases the moon is in as seen over a particular place, the length of a month is not standardized and can vary by as much as a day. Months can even start on different days in different places.

This is still a problem for many people, as debate continues over whether the month of Ramadan, which is a lunar month, should start based on local observations of the moon or if the phase observed over Mecca should be used.

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A day is the time that it takes for the Earth to turn once around its axis, or one day and one night. It is probably the earliest measurement of time used, although early people had no idea that the Earth was turning. They assumed that the Sun was moving, because it appeared to rise above the horizon each morning and disappear behind the opposite horizon at night.

A day is approximately the period of time during which the Earth completes one rotation around its axis. A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times. Days on other planets are defined similarly and vary in length due to differing rotation periods, that of Mars being slightly longer and sometimes called a sol.

In 1960, the second was redefined in terms of the orbital motion of the Earth in the year 1900, and was designated the SI base unit of time. The unit of measurement “day” was redefined as 86,400 SI seconds and symbolized d. In 1967, the second and so the day were redefined by atomic electron transition. A civil day is usually 86,400 seconds, plus or minus a possible leap second in Coordinated Universal Time (UTC), and occasionally plus or minus an hour in those locations that change from or to daylight saving time.

Day can be defined as each of the twenty-four-hour periods, reckoned from one midnight to the next, into which a week, month, or year is divided, and corresponding to a rotation of the earth on its axis. However, its use depends on its context; for example, when people say ‘day and night’, ‘day’ will have a different meaning: the interval of light between two successive nights, the time between sunrise and sunset; the time of light between one night and the next. For clarity when meaning ‘day’ in that sense, the word “daytime” may be used instead, though context and phrasing often makes the meaning clear. The word day may also refer to a day of the week or to a calendar date, as in answer to the question, “On which day?” The life patterns (circadian rhythms) of humans and many other species are related to Earth’s solar day and the day-night cycle.

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Many electronic devices nowadays have liquid crystal displays. Watches, music centres, calculators and even cars give information by means of liquid crystals. These are crystals, held inside cells that become opaque or change colour when they are heated. The circuits behind the display pass a voltage across the crystals, so that some of them change while the others remain the same. In this way, numbers, letters and symbols can be displayed.

Liquid crystal display (LCD), electronic display device that operates by applying a varying electric voltage to a layer of liquid crystal, thereby inducing changes in its optical properties. LCDs are commonly used for portable electronic games, as viewfinders for digital cameras and camcorders, in video projection systems, for electronic billboards, as monitors for computers, and in flat-panel televisions.

Liquid crystals are materials with a structure that is intermediate between that of liquids and crystalline solids. As in liquids, the molecules of a liquid crystal can flow past one another. As in solid crystals, however, they arrange themselves in recognizably ordered patterns. In common with solid crystals, liquid crystals can exhibit polymorphism; i.e., they can take on different structural patterns, each with unique properties. LCDs utilize either nematic or amectic liquid crystals. The molecules of nematic liquid crystals align themselves with their axes in parallel, Smectic liquid crystals, on the other hand, arrange themselves in layered sheets; within different smectic phases, the molecules may take on different alignments relative to the plane of the sheets.

The optical properties of liquid crystals depend on the direction light travels through a layer of the material. An electric field (induced by a small electric voltage) can change the orientation of molecules in a layer of liquid crystal and thus affect its optical properties. Such a process is termed an electro-optical effect, and it forms the basis for LCDs. For nematic LCDs, the change in optical properties results from orienting the molecular axes either along or perpendicular to the applied electric field, the preferred direction being determined by the details of the molecule’s chemical structure. Liquid crystal materials that align either parallel or perpendicular to an applied field can be selected to suit particular applications. The small electric voltages necessary to orient liquid crystal molecules have been a key feature of the commercial success of LCDs; other display technologies have rarely matched their low power consumption.

Our experience of time is that it flows forwards. It cannot be stopped or reversed but goes on in a continuous stream. We can only measure time in relation to other things that have a regular pattern: the rising of the Sun, the swinging of a pendulum or the vibration of a crystal.

Time provides us with a measure of change by putting dates on moments, fixing the durations of events, and specifying which events happen before which other events. In order to do that, some method of time measurement is needed. The science or art of the accurate measurement of time is known as chronometry (or, less formally, timekeeping). A similar concept, horology, usually refers to mechanical timekeeping devices or timepieces. Time can be measured either in terms of the absolute moment when a particular event occurs, or in terms of a time interval, i.e. the duration of a continued event.

There are two main methods used in the everyday measurement of time, depending on the accuracy required or the interval covered. A clock is a physical mechanism that counts the ongoing passage of time, and is mainly used for more accurate timekeeping and for periods of less than a day. A calendar is a mathematical abstraction used for calculating more extensive periods of time (i.e. longer than a day). Typically, both methods are used together to specify when in time a particular events occurs (e.g. 12:30PM on 16 December 2013). Even before such methods were devised, mankind has always used more informal methods for basic timekeeping, such as the cycle of the seasons, and of day and night, and the position of the Sun in the sky.

Chronology, as opposed to chronometry, is the science of arranging events in their order or sequence of occurrence in time, and is mainly used for studying the past. For convenience, events can be put into chronological groups, a process known as periodization. Chronology, periodization and the interpretation of the past are together known as the study of history.

The measurement of time involves the use of various different units of measurement, depending on the time scales and periods under consideration. These range from the almost infinitesimal units employed in physics, though the everyday units (e.g. seconds, minutes, hours, days, months, years, etc.), to the much larger units used in geological and cosmological time scales.

Different time standards, specifications for the measurement of time, have been in use throughout history, although modern globalization and scientific internationalism have led to at the adoption of highly accurate and largely universal standards of time measurement and central reference points.

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