planets and stars are gyroscopes-rotational inertia

Imagine that you have a bicycle wheel with excellent bearings mounted on an extended, two-foot axle. Suppose also that the tire is completely filled with lead, then balanced and spun up to about 1000 rpm. If you were to hold both ends of the axle while the wheel were spinning, you would discover that you could not turn the axle to point in a different direction. Although you could easily "slide" the axle through the air keeping it pointed in the same direction, you would be completely unable to change the direction in which the spin axis points!

Planets and stars are gyroscopes

Here’s a second experiment. Attach a top within a hoop. Now attach that hoop to rotate within a second hoop as in the picture above, so that the spin axes of the two hoops are at right angles. If the friction is negligible and the top spins at a high speed, the top’s axle will appear to slowly rotate end over end through a complete circle in 24 hours. Although it is the top that appears to rotate, the direction of the top’s spin axis in space is fixed relative to the stars. It is the Earth, carrying the room that the top is in with it, that is actually rotating around the top!!

How can this be? Well, Newton’s first law states that a body in uniform motion will tend to stay in motion unless acted upon by an outside force. This tendency of moving bodies to continue moving or to stay at rest is called "inertia." And spinning bodies -- the bicycle wheel, the top, and the Earth -- all have "rotational inertia:" the tendency of a spinning body’s spin axis to remain pointed in the same direction in space. As long as a spinning body is not acted upon by an outside force its spin axis will remain forever pointed in the same direction in space relative to the stars! This is how gyroscopes form inertial guidance systems for planes, submarines and spacecraft. These vehicles constantly refer to a gyroscope’s fixed and rigid orientation in space in order to navigate and maintain their bearings.

The spin of a planet or a star increases as it condenses from a cloud of gas and debris, just as a skater increases her spin by pulling in her arms. And since all spinning bodies have rotational inertia, the Earth’s spin axis always points in the same direction in space,* the North star as it orbits the Sun. Now the Earth’s equator is perpendicular to its axis, so the Earth’s equatorial plane also points in the same direction* in space as the Earth orbits the Sun. And since the Earth’s orbital plane (the ecliptic) is also directionally fixed in space, these two fixed planes (in red and blue below) intersect in a line that is also directionally fixed* in space, the orange-turquoise line of the equinoxes pointing to 0° Aries and 0° Libra. This point is illustrated on page 66 in "You and the Universe," and continued on this site on the page "The Ages of Pisces and Aquarius."

*Because the Sun and the Moon pull unevenly on the Earth (since the Earth is not a perfect sphere but bulges at its equator), the Earth’s spin axis does NOT always points in the same direction in space. Rather its direction in space paints a small circle on the celestial sphere which is completed every 26,000 years. This motion is known as the precession of the equinoxes.

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The author, his poetry and instruments		Virgo and Venus in "You and the Universe"