Astronomy for Everybody/Part 1/Chapter 3

We all know that a line running through any place on the earth in a north and south direction, is called the meridian of that particular place. More exactly, a meridian of the earth's surface is a semicircle passing from the north to the south pole. Such semicircles pass in every direction from the north pole, and one may be drawn so as to pass through any place. The meridian of the Royal Observatory at Greenwich is now adopted by most nations, our own included, as the one from which longitudes are measured, and by which in the United States and a considerable part of Europe the clocks are set.

Corresponding to the terrestrial meridian of a place is a celestial meridian which passes from the north celestial pole through the zenith, intersects the horizon at its south point, and continues to the south pole. As the earth revolves on its axis it carries the celestial as well as the terrestrial meridian with it, so that the former, in the course of a day sweeps over the whole celestial sphere. The appearance to us is that every point of the celestial sphere crosses the meridian in the course of a day.

Noon is the moment at which the sun passes the meridian. Before the introduction of railways, people used ​to set their clocks by the sun. But owing to the obliquity of the ecliptic and the eccentricity of the earth's orbit around the sun, the intervals between successive passages of the sun are not exactly equal. The consequence is that, if a clock keeps exact time, the sun will sometimes pass the meridian before and sometimes after twelve by the clock. When this was understood, a distinction was made between apparent and mean time. Apparent time was the unequal time determined by the sun; mean time was that given by a clock keeping perfect time month after month. The difference between these two is called the equation of time. Its greatest amounts are reached every year about the first of November and the middle of February. At the former time, the sun passes the meridian sixteen minutes before the clock shows twelve; in February, fourteen or fifteen minutes after twelve.

To define mean time astronomers imagine a mean sun which always moves along the celestial equator so as to pass the meridian at exactly equal intervals of time, and which is sometimes ahead of the real sun and sometimes behind it. This imaginary or mean sun determines the time of day. The subject will perhaps be a little easier if we describe things as they appear, imagining the earth to be at rest while the mean sun revolves around it, crossing the meridian of every place in succession. We thus imagine noon to be constantly travelling around the world. In our latitudes, its speed is not far from a thousand feet per second; that is to say, if it is noon at a certain place where we stand, it will one second afterward be noon about one thousand feet farther west, in ​another second a thousand feet yet farther west, and so on through the twenty-four hours, until noon will once more get back where we are. The obvious result of this is that it is never the same time of day at the same moment at two places east or west of each other. As we travel west, we shall continually find our watches to be too fast for the places which we reach, while in travelling east, they will be too slow. This varying time is called local or astronomical time. The latter term is used because it is the time determined by astronomical observations at any place.

Formerly the use of local time caused great inconvenience to travellers. Every railway had its own meridian which it ran its trains by; and the traveller was frequently liable to miss his train by not knowing the relation between his watch or a clock and the railway time. So in 1883, our present system of standard time was introduced. Under this system, standard meridians are adopted fifteen degrees apart, this being the space over which the sun passes in one hour. The time at which noon passes a standard meridian is then used throughout a zone extending seven or eight degrees on each side. This is called standard time. The longitudes which mark the zones are reckoned from Greenwich. It happens that Philadelphia is about seventy-five degrees in longitude, or five hours in time from Greenwich. More exactly, it is about one minute of time more than this. Thus the standard meridian which we use for the Middle ​States passes a little east of Philadelphia. When mean noon reaches this meridian, it is considered as twelve o'clock throughout all our Eastern and Middle States as far west as Ohio. An hour later, it is considered twelve o'clock in the Mississippi Valley. An hour later, it is twelve o'clock for the region of the Rocky Mountains. In yet another hour, it is twelve o'clock on the Pacific coast. Thus we use four different kinds of time, Eastern time, Central time, Mountain time, and Pacific time, differing from each other by entire hours. Using this time, the traveller only has to set his watch forward or back one hour at a time, as he travels between the Pacific and the Atlantic coast, and he will always find it correct for the region in which he is at the time.

It is by this difference of time that the longitudes of places are determined. Imagine that an observer in New York makes a tap with a telegraph-key at the exact moment when a certain star crosses his meridian, and that this moment is recorded at Chicago as well as New York. When the star reaches the meridian of Chicago, the observer taps the time of its crossing over his meridian in the same way. The interval between the two taps shows the difference of longitude between the two cities.

Another method of getting the same result is for each observer to telegraph his local time to the other. The difference of the two times gives the longitude.

In this connection, it must be remembered that the heavenly bodies rise and set by local, not standard, time. Hence the time of rising and setting of the sun, given in ​the almanacs, will not answer to set our watches by for standard time, unless we are on one of the standard meridians. One difference between these two kinds of time is that local time varies continuously as we travel east or west, while standard time varies only by jumps of one hour when we cross the boundaries of any of the four zones just described.

Midnight, like noon, is continually travelling round the earth, crossing all the meridians in succession. At every crossing it inaugurates the beginning of another day on that meridian. If it is Monday at any crossing, it will be Tuesday when it gets back again. So there must be some meridian where Monday changes to Tuesday, and where every day changes into the day following. This dividing meridian, called the "date line," is determined only by custom and convenience. As colonization extended toward the east and the west men carried their count of days with them. The result was that whenever it extended so far that those going east met those going west they found their time differing by one day. What for the westward traveller was Monday was Tuesday for the eastern one. This was the case when we acquired Alaska. The Russians having reached that region by travelling east, it was found that, when we took possession by going west, our Saturday was their Sunday. This gave rise to the question whether the inhabitants, in celebrating the festivals of the Greek Church, should follow the old or the new reckoning of ​days. The subject was referred to the head of the church at St. Petersburg, and finally to Struve, the director of the Pulkowa Observatory, the national astronomical institution of the empire. Struve made a report in favor of the American reckoning, and the change to it was duly carried out.

At the present time custom prescribes for the date line the meridian opposite that of Greenwich. This passes through the Pacific Ocean, and in its course crosses very little land—only the northeastern corner of Asia and, perhaps, some of the Fiji Islands. This fortunate circumstance prevents a serious inconvenience which might arise if the date line passed through the interior of a country. In this case the people of one city might have their time a day different from those of a neighbouring city across the line. It is even conceivable that residents on two sides of the same street would have different days for Sunday. But being in the ocean, no such inconvenience follows. The date line is not necessarily a meridian of the earth, but may deviate from one side to the other in order to prevent the inconvenience we have described. Thus the inhabitants of Chatham Island have the same time as that of the neighbouring island of New Zealand, although the meridian of 180° from Greenwich runs between them.