Why can't we see Venus at night when it is an inner planet?
I know that Venus is inner planet. I wonder how's the position of Sun, Earth, and Venus so we can see the existence of Venus on Earth. It seems we should see Venus through all the night because Venus is inner planet.
No, it is the outer planets that you can see all night. It is very easy to see if you draw a diagram of the orbits. Draw the Sun in the center and then the orbits of Earth, an inner planet, and an outer planet. Because the outer planet's orbit is outside the orbit of Earth, one can see it at any time of the night. For example, when the Earth lies between the planet and the Sun, then you can see the planet at zenith at midnight.
For the inner planet, it never gets very far away from the Sun. The further in the planet is, the closer to Sun (in the sky) it is. There is a maximum angle between the Sun and the planet in the sky and that happens when the planet subtends a right angle between Earth and the Sun. This is called the greatest elongation and in the case of Venus, it is something like 46 degrees. Hence, the maximum Venus can get from the Sun (either in the morning sky or the evening sky) is 46 degrees and hence Venus always sets in less than about 3 hours after sunset or rises no earlier than 3 hours before sunrise.
This is also the reason why Mercury is so hard to see; it is so close to the Sun that its maximum elongation is only about 19 degrees and so sets before an hour after sunset or rises no earlier than an hour before sunrise.
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