after which it becomes a "morning star," visible on the Eastern horizon shortly before Sunrise. From a
stationary point about 20° behind the Sun, it advances by direct motion to a superior conjunction with the Sun -
after which it becomes an "evening star," visible on the Western horizon shortly after Sunset.
As with the Moon, and all satellites with reference to the planet around which they revolve, Mercury always
turns the same face toward the Sun, except for a libration of 23° 7' in both directions: making a 47° zone of
temperate conditions, and 132° zones of perpetual heat and cold.
As seen from the Earth, Mercury presents phases, similar to those of the Moon, because of which its visible
size varies from 30' to 104' -- its crescent or new moon phase occurs at its inferior conjunction; its full moon
phase at its superior conjunction. Its minor elongation, about 18°, occurs 22 days before and after its inferior
conjunction; its major elongation, about 28°, 36 days before and after its superior conjunction. At its maximum
its visible size is 3¼ times its diameter. Two of Jupiter's moons are larger than the planet Mercury.
To locate Mercury in the evening sky, find in the ephemeris the dates of its major elongation before or after a
superior conjunction, and for 10 and 5 days before and after. Transfer into hours its R.A. and declination on
these five dates, and plot its course on a star map, making note its nearness to known bright stars. Tilt this map
toward the celestial North pole, and assume a horizon about 23° below the Mercury position. If weather
conditions permit it can be seen with the aid of a field glass - sometimes even with the naked eye.
Mercury made a transit across the face of the Sun on May 11, 1937.
Solar System bodies: Venus.
A brilliant planet reflecting a silvery-white light, it is the most brilliant object that illuminates the evening sky.
The Greeks associated it with Aphrodite. To the Romans, it was known as Lucifer, when the Morning Star: and
Vesper, when the Evening Star. To the Chaldeans it was Ishtar, and compared to the Sumerian virgin mother,
the "Lady of Heaven," and the goddess of fertility.
Like Mercury, Venus exhibits phases, from a large twin crescent at the Inferior Conjunction, when it is closest
to the Earth, and some- times visible in daylight if you know where to look for it, to a small round orb at the
Superior Conjunction, when it is on the opposite side of the Sun from the Earth. After the Superior Conjunction
it is an Evening Star, and thus is visible in the evening, sky after sun-down, setting later each evening until it
reaches its maximum elongation of about 47° - at which time it sets about 3 hours after the Sun.
Shortly thereafter it attains to its greatest brilliancy, then grows rapidly smaller as it again comes closer behind
the Sun, until at its Inferior Conjunction it becomes invisible. Thereafter it reappears on the other side of the
Sun and becomes again visible as the Morning Star. Its motion as a Morning Star, as measured from the Earth,
is slower because of its greater distance from the Earth: 26 million miles at the Inferior Conjunction, as
compared to 160 million miles at the Superior Conjunction.