Global Positioning System Reference
In-Depth Information
in the year 2102 CE. Much later in the future, around the year 7500 CE, the
closest star to true north will be Alpha Cephei, not Polaris. In 15000 CE it
will be Vega. We will have to wait another 24,000 years or so before Polaris
again becomes the North Star. Five thousand years ago, mariners who
looked to the North Star would have seen Thuban (Alpha Draconis). For
Pytheas, the North Star was Polaris, but it was several degrees further away
from true north than it is today.
16
Here is another aspect of rotation: the obliquity angle is directly respon-
sible for the four seasons that we experience. The rotational and orbital
geometry is illustrated in figure 1.12b. From this illustration we see why
the seasons arise. At di√erent places in the earth's orbit, the number of
hours of daylight is di√erent for any fixed position on the earth's surface
(say, your house). In summer you see the sun rise in the northeast (if you're
a northern hemisphere resident), move quite high into the sky, and then
set in the northwest. This happens when the rotation axis is inclined
toward the sun. In winter you see the sun rise in the southeast and cross
low in the sky to the southwest, where it sets—and this occurs when the
rotation axis is inclined away from the sun.
There are four lines of latitude that are so special that they are given
their own names. Their distinctiveness arises because of the obliquity of
the ecliptic (the 23
1
⁄
2
\ inclination angle). The Arctic Circle lies at latitude
66
1
⁄
2
\ (that is, 23
1
⁄
2
\ from true north); it marks the southernmost latitude in
the northern hemisphere at which the sun can be above the horizon 24
hours a day. Similarly, the Antarctic Circle is the northernmost latitude in
the southern hemisphere at which the sun can be above the horizon all day.
In other words, you have to be inside the Arctic or Antarctic Circles to ex-
perience a midnight sun. The Tropic of Cancer lies at latitude 23
1
⁄
2
\ north
and the Tropic of Capricorn at 23
1
⁄
2
\ south. In between these latitudes—in
''the tropics''—you can experience the sun directly overhead, vertically
above you. Outside the tropics, you cannot.
17
What if the obliquity were 0\—if the earth's rotation axis were aligned
16. Just to confuse you, I will point out here that the location of true north is also not
fixed.
Polar motion
—the movement of the earth's rotation axis, and so of the poles, across the
surface—amounts to a few meters per century. Polar motion is caused by movements
within the earth's mantle and core and by redistribution of mass following the last ice age. I
will have more to say about polar motion in chapter 2.
17. The seasons have nothing to do with earth's distance from the sun. The closest point
of approach (perihelion) occurs on or about January 4. Data for this section comes from
Burnham (1979, 3:2009). For a nonmathematical explanation of celestial phenomena, see
Kaler (1996).
