As they pass the sun going from east to west--that is, from the evening to the morning sky--the inferior planets go between us and the sun; and when they swing back from west to east, or from the morning to the evening sky, they pass on the side of the sun farthest away from us. When they are in a direct line with the earth and the sun they are said to be in conjunction. If at this point they are between us and the sun, it is inferior conjunction. If they are on the other side of the sun, they are said to be in superior conjunction. When the planet, as seen in the evening, has traveled toward the east as far from the sun as it will go during that particular revolution, it is said to be at its greatest eastern elongation. Elongation means simply apparent distance from the sun; hence, greatest eastern elongation is the greatest distance possible east of the sun from our point of view. Greatest western elongation, which we see in the morning before dawn, occurs when the planet is at its greatest apparent distance west of the sun. While apparently drawing near and then away from the sun, traveling obliquely up and down the evening and the morning sky, the planet has all the time been moving in one direction around the sun; but we could see the motion only as it appeared on the background of the sky. The planet is in reality just as far from the sun when it is in conjunction as at elongation. The difference is that we see it at a different angle, or from a different point of view. But it has not been at all times equally near to the earth. When an inferior planet is at greatest eastern elongation, it is, of course, east of the sun, and can be seen above the sun in the evening after sunset, and is an evening star. As it moves westward nearer and nearer to the sun, it is above the horizon a proportionately shorter time each evening, and is more and more obscured by the sun’s rays until it reaches inferior conjunction, when it is exactly between us and the sun, and hence at the point nearest to us. Here it becomes invisible, largely because it has its dark side toward us, but partly because the dazzling light of the sun entirely obscures it. Once in a while our relative positions are such that we see it pass like a black dot directly over the bright face of the sun. This is called a transit. But a transit does not occur at every inferior conjunction. It would so occur if the planet’s orbit and the earth’s were in exactly the same plane. But the small tilt that they have is sufficient to throw the planet, when it is passing the sun, into such an angle that it does not pass directly between the disc of the sun and us, but a little above or below. Thus transits are rather rare, though they occur periodically in the case of both Venus and Mercury, and will be spoken of elsewhere.