APPARENT MAGNITUDE or Diameter of the Sun, Moon, or any Planet, is the Quantity of the Angle that their Discs appear under, to an Observer on the Surface of the Earth.
The apparent Diameters of the Celestial Luminaries are subject to some Diversity. That of the Sun is observed to be least when he is in Cancer, and greatest when in Capricorn. See SUN.
In the Moon, there is a twofold Increase and Decrease of her apparent Diameter; the one, when she is in the Conjunction, and Opposition with the Sun; and the other in her Quadratures.
The greatest apparent Diameter of the Sun, according to Cassini, is 32’, 10", and the least 31’, 38’. According to de la Hire, the greatest is 32’, 43!", and the least 31’, 38!.
The greatest apparent Diameter of the Moon, according to Kepler, is 32’, 44", and the least 30’, 00". And according to de la Hire, the one is 33’, 30!", and the other 29’, 30!. See SUN and MOON.
The apparent Diameter of Saturn’s Ring, according to Huygens, is 1’, 18", when least. See SATURN. The apparent Diameters of the other Planets, see under the Article DIAMETER.
If the Distances of any two remote Objects, for example, two Planets, be equal, their true Diameters are proportional to the apparent ones; and if the apparent Diameters be equal, the true Diameters will be as the Distances from the Eye. Hence, when neither the Distances nor the apparent Diameters are equal, the true Diameters will be in a Ratio compounded of the direct Ratio of the Distances, and of the direct Ratio of the apparent Diameters.
There is a further very observable Difference between the apparent Magnitudes or Diameters of the Sun and Moon when in the Horizon, from what they are in the Meridian; the reason whereof has long perplexed the Philosophers. See MOON.
APPARENT MAGNITUDE
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- Written by: Ephraïm Chambers
- Category: Unclassified