Mercury and Venus Help

Introduction to Mercury and Venus

Twilight Stars

Neither Mercury nor Venus ever strays far from the Sun in the sky because both orbits lie entirely inside Earth’s orbit. Mercury rarely shows itself to casual observers; you have to know when and where to look for it, and it helps if you have a good pair of binoculars or a small telescope. Venus, in contrast, is at times the third brightest object in the sky, surpassed only by the Sun and the Moon.

Observation

The best time to look at Mercury is when it is at or near maximum elongation and when the ecliptic is most nearly vertical with respect to Earth’s horizon. Maximum elongations happen quite often with Mercury because it travels around the Sun so fast. But ideal observing conditions are rare.

Suppose that you live at temperate latitudes in the northern hemisphere. If Mercury is at maximum eastern elongation (the planet is as far east of the Sun as it ever gets) near the March equinox, it can be spotted with the unaided eye low in the western sky about a half hour after sunset. If Mercury is at maximum western elongation near the September equinox, look for the planet low in the eastern sky about a half hour before sunrise.

If you live in the southern hemisphere, the situation is reversed. If Mercury is at maximum eastern elongation near the September equinox, it can be spotted with the unaided eye low in the western sky about a half hour after sunset. If Mercury is at maximum western elongation near the March equinox, look for the planet low in the eastern sky about a half hour before sunrise.

Venus shows itself plainly much more often than does Mercury. In fact, this planet has sometimes been mistaken for an unidentified flying object (UFO) because it tends to “hover” in the sky and can be as bright as an aircraft on an approach path several miles away. With a light haze or with high-altitude, thin cirrostratus clouds, the planet can be blurred and the effect exaggerated. Nevertheless, ideal observing conditions occur under the same circumstances as with Mercury; Venus can be seen for several hours after sunset when it is at maximum eastern elongation and for several hours before sunrise when it is at maximum western elongation.

Phases

The inferior planets go through phases like the Moon. This fact was not known to astronomers until Galileo and his contemporaries first turned “spy glasses” to the heavens. The phases occur for the same reason the Moon goes through phases, and they can range all the way from a thin sliver of a crescent to completely full.

Figure 5-1 shows the mechanism by which an inferior planet attains its phases. The half-illuminated phases occur, in theory, at the points of maximum elongation, that is, when the angle between the planet and the Sun is greatest as seen from Earth. (In the case of Venus, this is not quite true because the thick atmosphere of that planet has a slight effect on the position of the twilight line.)

Figure 5-1. Phases of the inferior planets result from their relative positions with respect to Earth and the Sun.

The full phase of an inferior planet takes place at and near superior conjunction . When it is exactly at superior conjunction, the planet is obscured by the brilliance of the Sun and might even be eclipsed by our parent star. The new phase, which takes place at inferior conjunction , is usually invisible too, but not always. Sometimes Mercury passes so nearly in line between Earth and the Sun that it can be seen against the Sun’s disk when observed through a filtered telescope. On rare occasions, the same thing happens with Venus. When this happens, Mercury or Venus is said to transit the Sun.

Practice problems of this concept can be found at: Mercury and Venus Practice Problems