This imaginary line is called an axis. Earth spins around its axis, just as a top spins around its spindle. At the same time that the Earth spins on its axis, it also orbits, or revolves around the Sun.
This movement is called revolution. A pendulum set in motion will not change its motion, and so the direction of its swinging should not change. However, Foucault observed that his pendulum did seem to change direction. Since he knew that the pendulum could not change its motion, he concluded that the Earth, underneath the pendulum was moving. An observer in space will see that Earth requires 23 hours, 56 minutes, and 4 seconds to make one complete rotation on its axis.
But because Earth moves around the Sun at the same time that it is rotating, the planet must turn just a little bit more to reach the same place relative to the Sun. Hence the length of a day on Earth is actually 24 hours. At the equator, the Earth rotates at a speed of about 1, km per hour, but at the poles the movement speed is nearly nothing. The Earth belongs to both planes, so it lies on their intersection. We denote this line gg'. The Sun, wich is the centre of the Earth's annual orbit, belongs to the ecliptic plane too.
But it is rarely in the equator plane. When the Sun is in the North side of the equator plane, in Spring and Summer, on the Earth at the North Pole, one day lasts for six months.
When the Sun is on the South side of the equator plane, in Autumn and Winter, on the Earth at the North Pole, one night lasts for six months. So the Sun passes through the plane twice a year : at spring equinox, as seen from the Earth, the Sun is locatesd just on the g spot and at autumn equinox, it is located on the g' spot.
The geometric consequences of equinoxes are well-known : when the Sun crosses the equator plane, the bound of "night" and "day" on the Earth is a meridian circle the poles are on this circle and its plane is perpendicular to gg'. So on these dates, night and day each last about 12 hours. How to find the date of equinox with an ordinary equipment: Figure 2.
A simple vertical stick named gnomon by the Ancient Greeks is enough to find the day of equinox. During this day, the end of the shadow follows a straight East-West line. This line and the top of the stick locate the equator plane because on this date, the sun-rays that reach the Earth are on the equator plane. During the other days of the year, the end of the shadow is curved generally an hyperbolic arc. The previous observations plane perpendicular to polar axis or lines of planets give coarsely the directions of equator and ecliptic.
It is not enough to get precisely their intersection but if you can observe a lunar eclipse near equinox, it's all right! Equipment required : one photo of the Moon among the stars during an eclipse take this photo with a 28 or 50 mm focal length ; an equipment to mesure angles sextant, Orion's belt How to do it: We suppose that we know the direction of the ecliptic with preceding eclipses but not precisely the positions of g and g'spots that we want to locate.
First you must identify the zodiacal constellation on the photo in Autumn, it will probably be Pisces and in Spring Virgo. By the Moon, draw the line which represents eclipctic. This very night, the Moon is not on g or g' but it is at the opposite point of the Sun. Establish the scale of the photo. Compute the days from equinox to eclipse.
And as you know that the Sun goes on o ; in days, you can locate the opposite point of the Sun at the date of the equinox. If this eclipse takes place in Spring, this opposite point of the Sun is the Automn equinoctial point g'.
Measure the angle between this point and the brightest stars especially note the position of alpha Virgo also called Spica and you will be able to compare it with Ancient measurements.
The equinoctial precession: In about BC, Hipparcos , a great Greek astronomer, observed a lunar eclipse in Rhodos. He noticed that the brightest star in Virgo, Spica, rose 6 o ; before the g' spot.
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