Consider where Mercury is located, an average of 36 million miles from the Sun.
Since Mercury never travels far from the Sun, it is not visible at night.
But looking for it during the day is dangerous, because of its proximity to the Sun.
If you scan the sky, with binoculars, a telescope, or your naked eye, you are likely to locate the Sun before you find Mercury. This will probably cause serious eye damage or loss of sight.
With special sun filters on their telescopes, astronomers study Mercury during the day.
For the rest of us, the best time of day to locate this small planet is right after sunset and right before sunrise, when the Sun is still below the horizon.
Also, a few times each year, Mercury is at its greatest elongation, its greatest distance east or west of the Sun.
For instance, Mercury is farthest east on November 28, 1997, and June 9, 2000; it is farthest west on January 6, 1998, and August 14, 1999.
Within a few days of an elongation, Mercury is easier for astronomers to find.