So far, we have discussed the importance of the Earth'srotation on its axis. But what about the Earth's movementas it orbits the Sun? We refer to this motion asthe Earth's revolution around the Sun. The Earth takes365.242 days to travel around the Sun—almost a quarterof a day longer than the calendar year of 365 days. Everyfour years, this time adds up to nearly one extra day,which we account for by inserting a 29th day into Februaryin leap years. Further minor corrections—such asomitting the extra day in century years—are necessaryto keep the calendar on track.
The Earth's orbit around the Sun is shaped likean ellipse, or oval (Figure 1.17 ). This means that thedistance between the Earth and Sun varies somewhatthrough the year. The Earth is nearest to the Sun atperihelion , which occurs on or near January 3. It is farthestaway from the Sun at aphelion , on or near July 4.However, the distance between Sun and Earth variesonly by about 3 percent during one revolution becausethe elliptical orbit is shaped very much like a circle. Formost purposes we can regard the orbit as circular.Which way does the Earth revolve? Imagine yourselfin space, looking down on the North Pole. From thisviewpoint, the Earth travels counterclockwise aroundthe Sun (Figure 1.18 ). This is the same direction as theEarth's rotation.
MOTIONS OF THE MOON
The Moon rotates on its axis and revolves about theEarth in the same direction as the Earth rotates andrevolves around the Sun. But the Moon's rate of rotationis synchronized with the Earth's rotation so that one sideof the Moon is permanently directed toward the Earthwhile the opposite side of the Moon remains hidden.It was only when a Soviet spacecraft passing the Moontransmitted photos back to Earth in 1959 that we caughtour first glimpse of the far side.
The phases of the Moon are determined by the positionof the Moon in its orbit around the Earth, which inturn determines how much of the sunlit Moon is seenfrom the Earth. It takes about 29.5 days for the Moonto go from one full Moon to the next. In the twilightphoto of a moonlit scene in Figure 1.19 , the Moon isnearly full. From the way that the Sun illuminates theMoon as a sphere, it is easy to see that the Sun is downand to the right.
TILT OF THE EARTH'S AXIS
Depending on where you live in the world, the effectsof the changing seasons can be large. But why do weexperience seasons on Earth? And why do the hours ofdaylight change throughout the year—most extremelyat the poles, and less so near the Equator?Seasons arise because the Earth's axis is not perpendicularto the plane containing the Earth's orbit aroundthe Sun, which is known as the plane of the ecliptic.Figure 1.20 shows this plane as it intersects the Earth.
If we extend the imaginary axis out of the North Poleinto space, it always aims toward Polaris, the North Star.The direction of the axis does not change as the Earthrevolves around the Sun. Let's investigate this phenomenonin more detail.
THE FOUR SEASONS
Figure 1.21 shows the full Earth orbit traced on theplane of the ecliptic. On December 22, the north polarend of the Earth's axis leans at the maximum angle awayfrom the Sun, 23 ?°. This event is called the Decembersolstice , or winter solstice inthe northern hemisphere.At this time, the southernhemisphere is tilted towardthe Sun and enjoys strongsolar heating.
Six months later, on June21, the Earth has traveledto the opposite side of itsorbit. This is known as the June solstice , or summer solsticein the northern hemisphere. The north polar endof the axis is tilted at 23 ?° toward the Sun, while theSouth Pole and southern hemisphere are tilted away.The equinoxes occur midway between the solsticedates. At an equinox, the Earth's axis is not tiltedtoward the Sun or away from it. The March equinox(vernal equinox in the northern hemisphere) occursnear March 21, and the September equinox (autumnalequinox) occurs near September 23. The conditions atthe two equinoxes are identical as far as the Earth–Sunrelationship is concerned. The date of any solstice orequinox in a particular year may vary by a day or so,since the revolution period is not exactly 365 days.
The Sun's rays always divide the Earth into twohemispheres—one that is bathed in light and one thatis shrouded in darkness. The circle of illumination is thecircle that separates the day hemisphere from the nighthemisphere. The subsolar point is the single point on theEarth's surface where the Sun is directly overhead at aparticular moment.
At equinox, the circle of illumination passes throughthe North and South Poles, as we see in Figure 1.22.The Sun's rays graze the surface at both poles, so thesurfaces at the poles receive very little solar energy. Thesubsolar point falls on the Equator. Here, the anglebetween the Sun's rays and the Earth's surface is 90°,so that point receives the full force of solar illumination.At noon at latitudes in between, such as 40° N, theSun strikes the surface at an angle that is less than 90°.The angle that marks the Sun's elevation above thehorizon is known as the noon angle. Simple geometryshows that for equinox conditions the noon angle isequal to 90° minus the latitude, so that at 40° N, thenoon angle is 50°.
One important feature of the equinox is that dayand night are of equal length everywhere on the globe.You can see this by imagining yourself located at a pointon the 40° N parallel. As the world turns, you will bein daylight for exactly half the day and in night for theother half.
Now let's examine the solstice conditions in Figure1.23. The June solstice is shown on the left. Imaginethat you are back at a point on the lat. 40° N parallel.Unlike at equinox, the circle of illumination no longerdivides your parallel into equal halves because of the tiltof the northern hemisphere toward the Sun. Instead,daylight covers most of the parallel, with a smalleramount passing through twilight and darkness. For you,the day is now considerably longer (about 15 hours)than the night (about 9hours). Now step onto theEquator. You can see thatthis is the only parallelthat is divided exactly intotwo. On the Equator, daylightand nighttime hourswill be equal throughoutthe year.
The farther north you go, the more the effectincreases. Once you move north of lat. 66 ?°, theday con tinues unbroken for 24 hours. Looking atFigure 1.23 , we can see that is because the lat. 66 ?°parallel is positioned entirely within the daylight sideof the circle of illumination. This parallel is knownas the Arctic Circle. Even though the Earth rotatesthrough a full cycle during a 24-hour period, the areanorth of the Arctic Circle will remain in continuousdaylight. We can also see that the subsolar point is at alatitude of 23 ?° N. This parallel is known as the Tropicof Cancer. Because the Sun is directly over theTropic of Cancer at this solstice, solar energy is mostintense here.
The conditions are reversed at the December solstice.Back at lat. 40° N, the night is about 15 hours long,while daylight lasts about 9 hours. All the area southof lat. 66 ?° S lies under the Sun's rays, inundated with24 hours of daylight. This parallel is known as the AntarcticCircle. The subsolar point has shifted to a pointon the parallel at lat. 23 ?° S, known as the Tropic ofCapricorn.
We have carefully used the term daylight to describethe period of the day during which the Sun is abovethe horizon. When the Sun is not too far below thehorizon, the sky is still lit by twilight. At high latitudesduring the polar night, twilight can be several hourslong and provide enough illumination for many outdooractivities.
The solstices and equinoxes are four special eventsthat occur only onceduring the year. Betweenthese times, the latitude ofthe sub solar point travelsnorthward and southwardin an annual cycle, loopingbetween the Tropicsof Cancer and Capricorn.We call the latitude of thesubsolar point the Sun's
declination (Figure 1.24 ).As the seasonal cycle pro gresses, the polar reg ions thatare bathed in 24-hour daylight or shadowed in 24-hournight shrink and then grow. At other latitudes, thelength of daylight changes slightly from one day to thenext, except at the Equator, where it remains the same.In this way, the Earth experiences the rhythm of the seasonsas it continues its revolution around the Sun.