I am teaching science (and the rest of his curriculum) to my
twelve-year-old and have run into something quite fundamental
in astronomy
that I cannot explain to him: How is it possible to see Mercury
and Venus
at all? They are always closer to the Sun than the Earth so we
must look
towards the Sun to see them, but at night we cannot look in that
direction
and during the day the Sun is too bright. Looking at a diagram
(to scale)
of the solar system from "above" seems to show that
the the orbits of these
two planets can never intersect a possible line of sight from
the the
Earth's surface at night.
It's true that Venus and Mercury are always seen generally in the direction of the sun, but usually not in *exactly* the direction of the sun. Venus can be as much as 47 degrees away from the Sun in the sky, while Mercury can make it out to 23 degrees away. When the inferior planets are at their maximum distances from the Sun in the sky, we call that their "Greatest Elongation."
When Venus is at Greatest Elongation to the east of the Sun, for instance, we can see it in the sky when the Sun is down, but not more than 47 degrees below the horizon.
This is why Venus is the "Evening Star" or the "Morning Star." At various times of year, it can be seen either east (in the evening) or west (in the morning) of the Sun, but always in the Sun's general direction. So when Venus is significantly east of the Sun, you'll see the Sun set, and then Venus will appear 45 degrees or so above the sunset as the sky darkens. In another couple of hours, Venus will follow the Sun below the horizon.
Mercury is indeed extremely difficult to find. Since it only can reach 23 degrees of elongation away from the Sun, even at this maximum elongation, it is often caught in the Sun's glare at sunset. Even when visible, it is always very low in the sky and quickly sets during twilight.
Answers provided by HSU Astronomy Professor David Kornreich.