Earth and Its Orbit - The Science of Navigation - page 7

way—a phenomenon known as precession . The rotational axis direction is
not the same as the orbital axis direction. This di√erence, which is the
inclination of the equator to the orbit, is about 23 1 ⁄ 2 \, and it is responsible
for the seasons. The direction of the inclination changes slowly, a fact that
has consequences for navigation: Polaris, the Pole Star, is not always due
north. The shape of our planet's orbit about the sun (indeed, as Sir Isaac
Newton famously showed us, of all planets) is an ellipse. 2 For Earth, the
ellipse is very nearly a circle: the furthest and nearest distances to the sun
( aphelion and perihelion ) di√er from the average distance by only 1.6%. The
earth is not quite a sphere, though for most purposes we can approximate it
as one. In fact, the distance from the North Pole or the South Pole to the
center is about 6,353 km, and from a point on the equator to the center is
6,384 km (a shade short of 4,000 mi). The equatorial bulge of the earth—
due, of course, to the planet's rotation—was not known or understood
until the eighteenth century, as we will see. 3
Your planet has a satellite, the fifth largest in the solar system. By one
measure, however, our moon is way bigger than any other satellite: as a
fraction of the size of the planet it orbits. 4 This fact has consequences for
the exploration of the earth, especially in the days when exploration was
carried out in ships: the moon creates large and variable tides. Your planet
is dense, like the other inner planets of the solar system but unlike the
outer planets. The high density is due to the metal core, which is mostly
composed of iron, with some nickel. (Core material is denser near the
center of the earth for the same reason as inner planets are denser than
outer planets: gravitational pull.) The earth's core is solid from the center
out to a radius of about 1,200 km and liquid from 1,200 km out to about
3,400 km. Temperature increases from the surface of the earth toward the
center, which is why the outer core is molten iron, not solid iron. (The
inner core is solid, despite being hotter, because of the enormous pressure
that arises from gravity.) The existence of a liquid iron outer core has
consequences for navigation: Earth possesses a magnetic field.
2. In fact, Newton showed that the orbit is an exact ellipse only for a solar system with
one planet. The existence of other planets makes the analysis of planetary orbits, and the
orbits themselves, much more complicated. However, it is a very good approximation to say
that the orbit of each planet is an ellipse.
3. Earth orbital and dimensional data is available from many sources, such as Weast
(1973).
4. Not counting Charon, the main satellite of Pluto. Pluto is no longer classified as a
planet but is now considered to be a dwarf plant , one of five so far identified that orbit our
sun.