# .11 THE DAY. S it crosses the meridian at apparent noon. Before (ante) noon is thus a.m., while after (post) noon is p.m. FIGURE 7.

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1 Confirming Pages UNIT 7 PA RT I The Time of Day From before recorded history, people have used events in the heavens to mark the passage of time. The day was the time interval from sunrise to sunrise, and the time of day could be determined from how high the Sun was in the sky. As our ability to independently measure time has become more accurate, we have found that the apparent motions of the Sun across the sky are not as uniform as we once thought, and in this age of rapid travel and communications it no longer makes sense for each town to set its own time according to the Sun. With high-precision modern clocks we have even detected a gradual slowing of the Earth s spin! Each of these adjustments to our understanding of how to keep time provides an insight into the workings of astronomy. In this Unit we explore the motions of the Sun in detail. We use some basic ideas of day and night and celestial coordinates presented in Unit 5 and of the apparent motion of the Sun against the stars from Unit 6. If these ideas are unfamiliar, you may want to review those Units first. UNIT OUTLINE 7.1 The Day 7.2 Length of Daylight Hours 7.3 Time Zones 7.4 Daylight Saving Time 7.5 Leap Seconds THE DAY Meridian The length of the day is set by the Earth s rotation speed on its axis. One day is defined to be one rotation. However, we must be careful how we measure our planet s rotation. For example, we might use the time from one sunrise to the next to define a day. That, after all, is what sets the day night cycle around which we structure our activities. However, we would soon discover that the time from sunrise to sunrise changes steadily throughout the year as a result of the seasonal change in the number of daylight hours. A better time marker is the time it takes the Sun to move from its highest point in the sky on one day, what we technically call apparent noon, to its A.M. P.M. highest point in the sky on the next day a time interval that we call the solar day. Summer Local noon We often divide a day into a.m. and p.m., which stand for ante meridian and post meridian, respectively. Equinox The meridian is a line that divides the eastern and western halves of the sky. The meridian extends from the Winter point on the horizon due north to the point due south and passes directly through the zenith, the point exactly overhead. As the Sun moves across the sky (Figure 7.1), S it crosses the meridian at apparent noon. Before (ante) East West noon is thus a.m., while after (post) noon is p.m. FIGURE 7.1 Apparent solar time is what a sundial measures, and The Sun rises in the east, crosses the meridian at local noon, then sets in during the year this time may be ahead of or behind the west. This figure depicts the path of the Sun seen from the Northern clock time by as much as a quarter of an hour. This Hemisphere at the equinoxes and solstices. variation arises from the Earth s orbital characteristics 53 sch12133_ch07.indd 53

3 Unit 7 The Time of Day 55 FIGURE 7.3 Sequence of 24 pictures of the Sun taken from a spot close to the Arctic Circle near the summer solstice. The pictures were taken 1 hour apart and show the Sun circling the sky but never setting. ANIMATION Hours of daylight This variation in the number of daylight hours is caused by the Earth s tilted rotation axis. Remember that as the Earth moves around the Sun, its rotation axis points in very nearly a fixed direction in space. As a result, the Sun shines more directly on the Northern Hemisphere during its summer and at a more oblique angle during its winter. The result (as you can see in Figure 7.4) is that a large fraction of the Northern Hemisphere is illuminated by sunlight at any time in the summer, but a small fraction is illuminated in the winter. So as rotation carries us around the Earth s axis, only a relatively few hours of a summer day are unlit, but a relatively large number of winter hours are dark. On the first days of spring and autumn (the equinoxes), the hemispheres are equally lit, so that day and night are of equal length everywhere on Earth. If we change our perspective and look out from the Earth, we see that during the summer the Sun s path is high in the sky, so that the Sun spends a larger portion of the day above the horizon (Figure 7.1). This gives us not only more heat but also more hours of daylight. On the other hand, in winter the Sun s path across the sky is much shorter, giving us less heat and fewer hours of light (also see Unit 6). Sun Night No daylight 10.3 hr daylight 12 hr daylight 13.7 hr daylight 24 hr daylight Sunlight 24 hr daylight 13.7 hr daylight 12 hr daylight 10.3 hr daylight No daylight December 21 June 21 FIGURE 7.4 The tilt of the Earth affects the number of daylight hours. Locations near the equator always receive about 12 hours of daylight, but locations toward the poles have more hours of darkness in winter than in summer. In fact, above latitudes 66.5 the Sun never sets for part of the year (the midnight sun phenomenon) and never rises for another part of the year. At the equinoxes, all parts of the Earth receive the same number of hours of light and dark. (Sizes and separation of the Earth and Sun are not to scale.) Equator Night sch12133_ch07.indd 55

4 Confirming Pages 56 Part One The Cosmic Landscape 7.3 TIME ZONES 7. Because the Sun is our basic timekeeping reference, most people like to measure time so that the Sun is highest in the sky at about noon. As a result, clocks in different parts of the world are set to different times so that the local clock time approximately reflects the position of the Sun in the sky. Because the Earth is round, the Sun can t be overhead everywhere at the same time, so it can t be noon everywhere at the same time. By the late 1800s, with the increasing speed of travel and communications, it became confusing for each city to maintain its own time according to the position of the Sun in the sky. By international agreement, the Earth was therefore divided into 24 major time zones, centered every 15 of longitude, in which the time differs by one hour from one zone to the next. With this system, clocks in a time zone all read the same, and they are at most a half hour ahead of or behind what they would be if the time were measured locally. Many regions use local geographic features or political borders to define the boundaries between time zones rather than strictly following the longitude limits (see Figure 7.5). Authorities in a few countries and regions did not adopt this agreement, choosing instead to maintain a time standard that was closer to local time. For example, Newfoundland, India, Nepal, and portions of Australia are offset by 30- or 15-minute differences from the international standard. Across the lower 48 United States, the time zones are, from east to west, Eastern, Central, Mountain, and Pacific. Within each zone the time is the same everywhere and is called standard time. In the eastern zone the time is denoted Eastern Standard Time (EST), in the central zone it is denoted Central Standard Time (CST), and so on. If you travel across the United States, you reset your watch as you cross from one time zone to another, adding one hour for each time zone as you go east, and subtracting one hour for each time zone as you go west. If you travel through many time zones, you may need to make such a large time correction that you shift your watch past midnight. For example, if you could In Figure 7.5 the international date line has a very complex shape. If you were sailing north through the 11h time zone, how would your time and date change? FIGURE 7.5 Time zones of the world and the international date line. Local time = universal time + numbers on top or bottom of chart h h 3h 1h h 9h 3h 8h Pacific 7h Mountain 5h Eastern h +1h h +2h +3h +4h +8h +9h +5h45m +5h +6h +13h +14h 11h +11h +13h 9h 5h 4h 10h +3h 45m sch12133_ch07.indd h +3h +4h 0h +9 +9h +5h +1h 1h 6h Central +5 +4h 4h International Date Line +4 +7h 9h 10h +3 Prime meridian Concept Question 1 3h +9h +8h +11h Locations where time differs by a fraction of an hour from standard

5 Confirming Pages Unit 7 The Time of Day Concept Question 2 In Jules Verne s story Around the World in 80 Days, the travelers discover that although they have experienced 80 days and nights, they still have one more day before 80 days will have passed in London. How can this be? 57 travel westward quickly enough that little time elapsed, setting your watch back each time you crossed a time zone, you could end up at your starting point with your watch turned backward by 24 hours. But you would not have traveled back in time! When you cross longitude 180 (roughly down the middle of the Pacific Ocean), you add a day to the calendar if you are traveling west and subtract a day if you are traveling east. For example, you could celebrate the New Year in Japan and take a flight after midnight to Hawaii, where it would still be the day before, so you could celebrate the New Year that night too! The precise location where the day shifts is called the international date line (Figure 7.5). It generally follows 180 longitude but bends around extreme eastern Siberia and some island groups to ensure that they keep the same calendar time as their neighbors. The nuisance of having different times at different locations can be avoided by using universal time, abbreviated as UT. Universal time is the time kept in the time zone containing the longitude zero, which passes through Greenwich, England. By using UT, which is based on a 24-hour system to avoid confusion between a.m. and p.m., two astronomers at remote locations can make a measurement at the same time without worrying about what time zones they are in. 7.4 DAYLIGHT SAVING TIME 7. International Date Line Prime meridian FIGURE 7.6 Regions where daylight saving time is observed for some portion of the year. In many parts of the world, people set clocks ahead of standard time during the summer months and then back again to standard time during the winter months (Figure 7.6). This has the effect of shifting sunrise and sunset to later hours during the day, thereby creating more hours of daylight during the time when most people are awake. Time kept in this fashion is called daylight saving time in the United States. In some other parts of the world it is called Summer Time. Locations where northern summer daylight saving time is observed Locations where southern summer daylight saving time is observed sch12133_ch07.indd 57

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