Bulletin of the American Meteorological Society

WE GET NEARER THE SUN AS WINTER COMES

By Isabel M. Lewis, of the U. S. Naval Observatory

(Science Service)

When winter begins in the northern hemisphere on December 22, at 4.08 a. m., Eastern Standard Time, we will be three million miles nearer to the sun than we were at the beginning of summer last June. The earth reaches perihelion, the point in its orbit nearest the sun, on January 3. It will then be about 91,346,000 miles from the sun. It was in aphelion, the point in its orbit farthest from the sun, on July 4th and its distance from the sun was then about 94,346,000 miles. The mean average distance of the earth from the sun is about 92,897,000 miles, so the least and greatest distance differ about 1,500,000 miles from the mean.

As our seasonal changes depend far more upon the constantly changing angle at which the sun's rays strike the surface, caused by the tilt of the earth's axis to the plane of its orbit, than upon comparatively small changes in the distance of the earth from the sun we feel no great effect from this change in distance.

Our winter begins when the sun reaches its greatest distance south of the equator. This time can be computed to the minute and it never varies much over a day from year to year. At that time the sun passes at noon through the zenith of all points in 23 degrees 27 minutes South Latitude, and summer begins in the southern hemisphere at the same instant that winter begins in the northern hemisphere. The sun then starts its northward journey and crosses the equator once more at the beginning of spring. Since the summer of the southern hemisphere occurs when the earth is nearest the sun and the winter when it is farthest from the sun we would naturally expect to find the summers in the southern hemisphere hotter than our summers and the winters colder than our winters. But the earth is moving most rapidly when it is nearest the sun and most slowly when it is farthest from the sun. As a result the summer of the southern hemisphere is about seven days shorter than our summer and while the heat of the summer is somewhat more intense in the southern hemisphere it is received for a shorter period. As a result there is very little difference in the total amount of heat received during the summer season in the two hemispheres.

The winters of the southern hemisphere are both longer and colder than the winters of the northern hemisphere as they occur when the earth is farthest from the sun and is, at the same time, moving most slowly in its orbit. In about ten thousand years these conditions will be reversed owing to certain slow changes now taking place in the form and position of the earth's orbit. The winters in our hemisphere will then be longer and colder than the winters of the southern hemisphere.

Some geologists go so far as to say that the glacial periods are caused by this alteration in the relative intensity of winter in the two hemispheres. It is certainly true that at the present time the south polar cap extends farther into the temperate zone than the north polar cap.

BULLETIN

OF THE

AMERICAN METEOROLOGICAL SOCIETY

Published for the American Meteorological Society at Easton, Pennsylvania, and Worcester, Massachusetts, U. S. A.

INDEX TO VOLUME II, 1921

As the BULLETIN is not a medium for the first publication of original contributions to meteorological science or its applications, but rather a bulletin of current progress in meteorology and of the affairs of the Society, authors' names have not been included in the index. The great number of short abstracts has made it impossible to publish a comprehensive index with the funds available; a much more satisfactory index of current meteorology is that for the MONTHLY WEATHER REVIEW, from which some synopses have been reprinted in the BULLETIN. Items not alphabetically classified may be found in most cases by referring to more general subjects, such as "Meteorology," "Weather," and especially by looking up the page references to the reports from the committee in whose province the subject would fall: see under "Committees."

Oct.-Nov.
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CORRIGENDA.

Page numbers should be 121-135 instead of 1-15.

1st line: under "clouds" "1921" should read "1920."
10th line: change "East Indies" to "West Indies."
11th line: change "Zomá” to “Zonia."

4th line from bottom: change "brown mountain" to "Brown
Mountain."

Page 139 Footnote: Wire from which 9 inch piece of ice fell was probably 1-inch in diameter.

Page 142 20th line from bottom: "three" should be "three thousand."