From the Northern Hemisphere, Mercury is best seen at an evening apparition during March or April, and at a morning one in September or October. (In the Southern Hemisphere, the best evening apparitions come in September or October, and morning ones in March or April.) At these seasons.
Mercury's orbit tilts most steeply to the horizon, so it reaches maximum separation from the Sun and best visibility.
At a good evening apparition, you can follow Mercury tor about two weeks before the date of greatest elongation, and a week after; for morning apparitions, it is a week before and two weeks after.
Mercury's apparent size varies according to its phases. When Mercury first appears in the evening sky, it is coining around the tar side of its orbit toward us. Seen in a telescope, it presents a warm-gray disk that looks like a tiny gibbous
Moon devoid of features. At greatest eastern elongation, it appears half lit. As it approaches inferior conjunction, it grows in size while its illuminated portion shrinks to a crescent.
After passing between Earth and the Sun, it repeats the phases in reverse order, finally reaching superior conjunction and starting the cycle anew.
Mercury typically presents a small disk that shimmers from the poor seeing near the horizon. Occasionally, with good seeing, you can use 200x magnification or more, yielding a f view like that of the naked-eye Moon. For better seeing, try viewing in twilight, when
Mercury is higher in the sky, and there is less contrast between sky and planet. This strategy works best for a morning apparition—you locate the planet low in a relatively dark sky and track it as it rises and the sky brightens.
If your telescope has an equatorial mounting, you can observe Mercury in full day-light. On a clear day, find the coordinates of both the Sun and Mercury in an almanac, magazine, or computer program. Work out the difference in their positions, then center the Sun, and step off the distance to Mercury. Scan with a low-power eyepiece to locate the planet.