MERCURY, ♁ in Astronomy. (See ASTRONOMY.) This planet is brightest between his elongations and superior conjunction, very near to which last he can generally be seen. He becomes invisible soon after he has found his elonga-
tion, going towards his inferior conjunction, and becomes visible again a few days before his next elongation. The brightness of this planet alters sometimes very considerably in twenty-four hours. It has been observed when less than three degrees distant from the sun, and may, perhaps, sometimes be seen even in conjunction with it. Mercury and Venus appear brightest and most beautiful in the opposite parts of their orbits; the former between his elongations and superior conjunction, and the latter between her elongations and inferior conjunction. Therefore, Venus is seen in great perfection as a crescent, particularly in her inferior conjunction, whilst Mercury is seldom seen in such perfect phases. Mercury should be always observed on or near the meridian. When farthest from the sun, he always appears with a very faint light; and when he has a great south declination, or the atmosphere is not perfectly clear, he can seldom be seen in those parts of his orbit, where he only begins to recover his brightness, or where it is much diminished. He has frequently been seen on the meridian even with a telescope of small power; and it appears from the above statement that he may be obscured in a clear day rather more than half his orbit, or nearly one hundred and fourscore days in the year.