How Long Does It Take for a Star to Be Direcctly Overhead Again

Learning Objectives

By the end of this section, you will be able to:

• Explain the difference between the solar 24-hour interval and the sidereal day
• Explicate mean solar time and the reason for fourth dimension zones

The measurement of fourth dimension is based on the rotation of Globe. Throughout most of human history, fourth dimension has been reckoned by positions of the Dominicus and stars in the sky. Only recently have mechanical and electronic clocks taken over this function in regulating our lives.

The Length of the Day

The most fundamental astronomical unit of time is the day, measured in terms of the rotation of Earth. In that location is, however, more than one way to define the day. Usually, we recollect of it as the rotation period of Earth with respect to the Sun, called the day. After all, for most people sunrise is more important than the rise time of Arcturus or some other star, and then nosotros set our clocks to some version of Sun-fourth dimension. However, astronomers also use a sidereal twenty-four hour period, which is defined in terms of the rotation catamenia of World with respect to the stars.

A solar solar day is slightly longer than a sidereal twenty-four hours because (as you tin can see from Figure one) World not merely turns but also moves forth its path around the Sunday in a day. Suppose we showtime when Earth's orbital position is at day i, with both the Sun and some distant star (located in the direction indicated by the long white arrow pointing left), directly in line with the zenith for the observer on Globe. When Earth has completed one rotation with respect to the afar star and is at day 2, the long pointer once again points to the aforementioned distant star. However, discover that considering of the motility of Earth forth its orbit from day i to 2, the Sun has not withal reached a position above the observer. To complete a solar mean solar day, Earth must rotate an boosted amount, equal to 1/365 of a full turn. The time required for this extra rotation is ane/365 of a day, or about four minutes. Then the solar day is virtually four minutes longer than the sidereal day.

Effigy ane: Difference Between a Sidereal Day and a Solar Solar day. This is a top view, looking down as Earth orbits the Sun. Because Earth moves effectually the Sunday (roughly i° per day), later one complete rotation of Earth relative to the stars, we do non run into the Dominicus in the same position.

Because our ordinary clocks are set to solar time, stars rise 4 minutes before each day. Astronomers adopt sidereal time for planning their observations because in that system, a star rises at the same time every day.

Example 1: Sidereal Fourth dimension and Solar Time

The Sun makes a complete circumvolve in the heaven approximately every 24 hours, while the stars make a complete circumvolve in the sky in 4 minutes less time, or 23 hours and 56 minutes. This causes the positions of the stars at a given time of solar day or night to change slightly each day. Since stars ascent 4 minutes earlier each 24-hour interval, that works out to almost 2 hours per month (four minutes × xxx = 120 minutes or 2 hours). So, if a detail constellation rises at dusk during the winter, you tin can be sure that past the summer, it will rise about 12 hours earlier, with the sunrise, and information technology will not exist so easily visible in the night sky. Allow'south say that tonight the brilliant star Sirius rises at 7:00 p.m. from a given location then that by midnight, it is very loftier in the sky. At what time will Sirius rise in 3 months?

Check Your Learning

If a star rises at 8:30 p.1000. tonight, approximately what time will information technology ascent ii months from now?

Credible Solar Fourth dimension

We can ascertain apparent solar time equally time reckoned past the actual position of the Sun in the sky (or, during the dark, its position below the horizon). This is the kind of time indicated by sundials, and it probably represents the earliest measure of time used by ancient civilizations. Today, nosotros adopt the middle of the dark every bit the starting point of the 24-hour interval and measure time in hours elapsed since midnight.

During the offset half of the solar day, the Sun has not yet reached the meridian (the great circle in the sky that passes through our zenith). We designate those hours every bit before midday (ante meridiem, or a.thousand.), before the Sun reaches the local height. Nosotros customarily start numbering the hours subsequently apex over once more and designate them by p.m. (postal service meridiem), after the Sun reaches the local meridian.

Although apparent solar time seems simple, it is not really very convenient to use. The exact length of an credible 24-hour interval varies slightly during the year. The eastward progress of the Sun in its almanac journey around the sky is non compatible because the speed of Earth varies slightly in its elliptical orbit. Another complication is that Earth's axis of rotation is not perpendicular to the plane of its revolution. Thus, apparent solar time does not advance at a uniform rate. After the invention of mechanical clocks that run at a uniform rate, information technology became necessary to carelessness the apparent solar day as the fundamental unit of measurement of time.

Mean Solar Time and Standard Time

Instead, we tin consider the mean solar time, which is based on the average value of the 24-hour interval over the class of the yr. A mean solar day contains exactly 24 hours and is what nosotros utilize in our everyday timekeeping. Although mean solar time has the advantage of progressing at a uniform rate, it is still inconvenient for applied utilise because it is adamant by the position of the Sun. For instance, noon occurs when the Lord's day is overhead. But considering we live on a round Earth, the exact time of noon is unlike every bit you modify your longitude by moving e or w.

If mean solar fourth dimension were strictly observed, people traveling e or west would accept to reset their watches continually as the longitude changed, but to read the local mean fourth dimension correctly. For instance, a commuter traveling from Oyster Bay on Long Island to New York Urban center would have to adapt the fourth dimension on the trip through the East River tunnel because Oyster Bay fourth dimension is actually near ane.6 minutes more than advanced than that of Manhattan. (Imagine an airplane trip in which an obnoxious flight bellboy gets on the intercom every infinitesimal, maxim, "Please reset your watch for local mean fourth dimension.")

Until almost the end of the nineteenth century, every city and town in the United States kept its own local hateful time. With the development of railroads and the telegraph, however, the need for some kind of standardization became axiomatic. In 1883, the Usa was divided into four standard fourth dimension zones (now half-dozen, including Hawaii and Alaska), each with ane system of fourth dimension within that zone.

By 1900, most of the earth was using the system of 24 standardized global fourth dimension zones. Within each zone, all places go along the aforementioned standard time, with the local mean solar time of a standard line of longitude running more or less through the middle of each zone. Now travelers reset their watches just when the time modify has amounted to a full hour. Pacific standard time is 3 hours earlier than eastern standard time, a fact that becomes painfully obvious in California when someone on the Eastward Coast forgets and calls you lot at 5:00 a.yard.

Globally, almost all countries take adopted one or more standard time zones, although one of the largest nations, India, has settled on a half-zone, being 5.5 hours from Greenwich standard. Also, several large countries (Russia, Communist china) officially use merely ane fourth dimension zone, so all the clocks in that country go on the same time. In Tibet, for case, the Dominicus rises while the clocks (which keep Beijing time) say it is midmorning already.

Daylight saving time is just the local standard time of the place plus ane hour. It has been adopted for spring and summertime employ in about states in the Us, every bit well as in many countries, to prolong the sunlight into evening hours, on the apparent theory that it is easier to change the time by regime action than it would be for individuals or businesses to adjust their own schedules to produce the same effect. It does not, of course, "save" whatever daylight at all—because the corporeality of sunlight is not adamant by what we do with our clocks—and its observance is a point of legislative debate in some states.

The International Appointment Line

The fact that time is e'er advancing as y'all motion toward the due east presents a problem. Suppose you travel east effectually the world. You laissez passer into a new fourth dimension zone, on the average, virtually every 15° of longitude you travel, and each time you dutifully prepare your lookout ahead an hour. By the time y'all have completed your trip, you have set your watch alee a full 24 hours and thus gained a day over those who stayed at domicile.

The solution to this dilemma is the International Date Line, set by international understanding to run approximately along the 180° tiptop of longitude. The appointment line runs downward the centre of the Pacific Bounding main, although it jogs a bit in a few places to avoid cutting through groups of islands and through Alaska (Effigy two). By convention, at the date line, the date of the calendar is inverse by one day. Crossing the date line from west to e, thus advancing your time, you lot compensate by decreasing the date; crossing from east to due west, you lot increase the engagement by one day. To maintain our planet on a rational system of timekeeping, we just must take that the date will differ in different cities at the same time. A good example is the date when the Imperial Japanese Navy bombed Pearl Harbor in Hawaii, known in the The states as Sunday, December 7, 1941, just taught to Japanese students as Monday, December eight.

Figure 2: Where the Date Changes.The International Date Line is an arbitrarily fatigued line on World where the appointment changes. So that neighbors do not accept different days, the line is located where Globe's surface is mostly h2o.

key concepts and summary

The basic unit of astronomical time is the day—either the twenty-four hours (reckoned by the Dominicus) or the sidereal 24-hour interval (reckoned by the stars). Credible solar time is based on the position of the Dominicus in the sky, and hateful solar time is based on the average value of a 24-hour interval during the year. Past international understanding, nosotros define 24 fourth dimension zones around the globe, each with its ain standard time. The convention of the International Date Line is necessary to reconcile times on different parts of Earth.

Glossary

apparent solar time: time as measured by the position of the Dominicus in the sky (the time that would be indicated by a sundial)

International Date Line: an arbitrary line on the surface of Earth near longitude 180° across which the engagement changes by one day

mean solar time:time based on the rotation of Earth; mean solar time passes at a constant charge per unit, unlike apparent solar time

sidereal day: Globe'south rotation period as defined by the positions of the stars in the sky; the fourth dimension between successive passages of the same star through the meridian

day: Earth's rotation period as defined by the position of the Sun in the sky; the time betwixt successive passages of the Lord's day through the meridian

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Source: https://courses.lumenlearning.com/astronomy/chapter/keeping-time/

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