Which three kinds of motion does the earth have?
Answers
Answer:
The Earth's Three Motions. The Earth turns (rotation around the polar axis), goes along on its orbit (revolution around the Sun), swings smoothly as un unbalanced spinning top (equinoctial precession).
Explanation:
As long as you live on the Earth, these motions remain imperceptible. So during the ancient times, the Greek astronomers thought that the sky was moving around a motionless Earth (in the opposite direction of real Earth motions).
The next pages don't tell you which one of Earth and Sky is moving. They only show you how you can see the effect of these motions in the stars.
It's easy to detect the rotation (an activity for 12 years old, Astronomy On-Line basic level), it's more difficult to detect the revolution (Astronomy On-Line middle level) and much more to detect the precession (here, an activity for 16 years and more, Astronomy On-Line advanced level).
The Earth swings smoothly as an unbalanced spinning top
Equinoxes and g and g' spots:
Let's sum up results of previous exercises.
The rotation of the Earth around its axis establishes on the sky the circle of the equator plane.
The motion around the Sun establishes the ecliptic circle in the sky.
The polar axis is not perpendicular to the ecliptic plane, so the equator and ecliptic planes make a dihedral angle about 23.5o; wide.
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'sannualorbit, 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:
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).
We can see such kinds of arcs on sundials, corresponding to the date.
How to locate easily the (gg') line in the sky:
As this line crosses the Earth, we have to find only one spot on it : g or g'.
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...) which gives the scale of the photo; and
a calendar to have the date of this eclipse ; and a gnomon or any kind of sundial to get the date of equinox.
Answer:
periodic motion,rotation motion,revolution motion
Explanation:
hope its helpful