How is the position of the full moon in relation to earth and the sun differnt from the position of the full venus in relation to earth and the sun? does the apparent size of the moon change with its phases?
2007-03-18 15:36:11 · 3 answers · asked by aDam 1 in Science & Mathematics ➔ Astronomy & Space
It is purely a matter of what part of the body in question is being illuminated by the sun, and the position of Earth in relation to it.
If you could see a movable model of what is happening it is quite obvious.
Venus shows phases because its orbit around the sun is inside Earth's orbit. So, if Venus position in its orbit is 90 degrees from our position, you will see a "half" Venus.
If it is diamterically opposite Earth on the far side of the sun, and assuming the glare of the sun is not in the way, you will see a "Full" Venus.
If it is directly between Earth and the sun, the sun is shining on the side facing the sun, and Venus will be invisible to us.
In the last scenario, rarely, Venus actually passes exactly between the earth and sun, and its tiny disk can be seen as a black dot crossing the sun. It's called a "Transit of Venus".
The Moon on the other hand goes from New to Full to New again in the time it takes it to orbit the Earth. It is not hard to visualise if you can see a model or a computer simulation.
2007-03-18 16:04:10 · answer #1 · answered by nick s 6 · 0⤊ 0⤋
The Moon is full when it is "behind" us: in the opposite direction from the Sun. i.e., when we face the full moon, we must turn our back to the Sun.
The Moon is always (approximately) at the same distance from us, so it always appear to be the same size, whatever its phase. Because its orbit is not exactly circular, there is a tiny difference. However, the size is directly related to the position of the Moon relative to the perigee and apogee of its orbit (points closest to Earth and furthest from Earth) which are not connected to the phases.
Moon's next perigee is on Monday March 19th (357,814 km centre-to-centre) and next apogee is on April 3rd (406,329 km). These happen to match New Moon (19th) and Full Moon. However, in August, they will match the First and Last Quarters.
The difference is a little more than 13%, giving a difference of a little more than 13% in the apparent size of the Moon. This can be measured with simple tools, such as a marine sextant.
Full Venus occurs when Venus is completely on the other side of the Sun from us: when we face full Venus, we must also face the Sun and look beyond the Sun. Because the orbit of Venus is not exactly on the same plane as our orbit, it is possible to still see it (with proper instruments) when it is on the other side of the Sun.
At that time, Venus is 1.72 astronomical units from us. When we see Venus as a thin crescent, it is much closer to us: a little over 0.28 astronomical units. The apparent extent of the thin crescent Venus is 6 times the apparent size of "Full Venus". That is a big difference.
2007-03-18 23:14:34 · answer #2 · answered by Raymond 7 · 1⤊ 0⤋
Venus revolves around the sun.
The earth revolves around the sun while the moon revolves around the earth and the sun.
Some times the moon can seem bigger but it may be due to comparing the moon with something on the horizon.
2007-03-19 10:44:10 · answer #3 · answered by Billy Butthead 7 · 0⤊ 0⤋