Planetary Phases and Motion

Venus is currently rising before the Sun at dawn.

By Ian Ridpath

"The phases of the Moon are very familiar to us, but the inner planets, Mercury and Venus, can also show phases as they move around their orbits. In fact, it was Galileo's discovery of the phases of Venus in the early seventeenth century that confirmed the planets orbit the Sun and not the Earth.

With one exception, the outer planets do not show appreciable phases because, as seen from Earth, they always appear fully illuminated by the Sun. The exception is Mars, which can appear slightly gibbous (i.e. not quite circular) at certain positions in its orbit, when it lies at an angle of about 90 degrees from the Sun, relative to the Earth.

As seen through a telescope, both Mercury and Venus display a full range of phases, but Venus is much the easier of the two planets to see since it is brighter and farther away from the Sun.

When Venus is at crescent phase, in the morning or evening sky, it is close to Earth and appears large enough for the phase to be visible through binoculars.

As Venus moves away from the Sun in the morning sky, the phase increases from a crescent, reaching half phase at the time of greatest elongation. The distance between Venus and Earth is increasing, so its disc is becoming smaller. Continuing on around its orbit, Venus increases in phase until, at superior conjunction, it is fully illuminated - but then it is on the far side of the Sun and cannot be seen.

After superior conjunction, Venus emerges into the evening sky and begins to decrease in phase as it approaches Earth, becoming a slim crescent again as it moves towards inferior conjunction. Venus is at its brightest about five weeks before and after inferior conjunction, when its phase is about 27%, i.e. a thick crescent."
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7/15 Tweet~ Normally it takes less then 10 min to drive over the Bay Bridge, but because of an accident last night, it took an hour and a half- Mercury retrograde.