Mercury in the Sky and in our Telescopes

Because Mercury orbits inside the orbit of Earth, many of the same basic rules that apply to the movement of Venus though the sky also apply to the movements of Mercury. Mercury just does everything much faster than Venus does. From behind the Sun, the planet first appears in the evening sky pulling rapidly away from the Sun. Because the planet's orbit is considerably more elongated than is that of Venus, Mercury's angular distance from the Sun at greatest elongation can range anywhere from 18 to 28 degrees. The planet then rapidly closes back in on the Sun, within a matter of a few weeks and passes through inferior conjunction. Within a few days if circumstances are favorable, the planet will rapidly appear in the morning sky, pulling out to its greatest angular distance in just about three weeks. The planet will then circle around behind the Sun coming back to superior conjunction about 120 days after previously being there.That means that Mercury overtakes and passes Earth three times every year. By comparison, Venus needs about 540 days to go from one superior conjunction to the next. Mercury will run through the same range of phases as Venus does. There are however many differences in the way Mercury behaves from Venus.

The first is the unfortunate coincidence for northern hemisphere observers is that when Mercury reaches greatest elongation at aphelion; it does so when the viewing geometry is at its most unfavorable. This occurs in March for morning apparitions and September for evening apparitions. When Mercury reaches greatest elongation in March for evening apparitions or September for morning apparitions, it is always near perihelion. So this gives rise to a startling contradiction. An evening apparition of Mercury is much more favorable in March, when the planet strays only 18 degrees from the Sun. That 18 degrees is almost vertically above the Sun and Mercury is in the sky for as much as an hour and a half after sunset. When the planet reaches greatest elongation in the evening sky in September, it will stretch to 28 degrees away from the Sun,but that 28 degrees is almost straight along the horizon. The planet never rises out of the horizon haze and sets within 40 minutes after the Sun.

While Venus is brilliantly reflective, Mercury is a dullard. The planet has no atmosphere, no bright reflective surface features such as oceans and much of the surface is made of dark materials that are likely volcanic in nature. While Venus reflects 80% of the light that strikes it back to space, Mercury is more like the Moon and reflects back only about 7% of the light that strikes it. Mercury is also less than half the size of Venus and so never has as much surface area to show as Venus does. Venus at its smallest apparent size is nearly as big as Mercury can ever appear at its largest.

In a telescope, Mercury runs through phases the same way Venus does. Mercury is much smaller than Venus is. At superior conjunction, the planet's disk is slightly less than four arc seconds across and by the time the planet is lost near inferior conjunction, its disk is between 10 and 12 arc seconds across. So as it draws nearer to us, Mercury's disk increases in size by a factor of two and half from the time it first appears to the time it disappears. By comparison, Venus disk increases by a factor of about six over the same period of time. Because of this, Venus reaches its point of greatest illuminated extent when it is a thick crescent. Mercury is at its greatest illuminated extent within a couple of weeks of superior conjunction. To illustrate, a fully illuminated Mercury four arc seconds across shows more illuminated area than a one-third illuminated Mercury eight seconds across. This gives Mercury a property unique in all the universe. By comparison, a fully illuminated Venus is nine arc seconds across. When it is one-third illuminated, it is 45 seconds across. This explains the mystery of Mercury's changing brightness that we illustrated in Chapter 1. Mercury is the only object in all the heavens that grows fainter as it gets closer! After passing superior conjunction, Mercury will shine at anywhere between magnitude -2.0 and -1.5 depending upon its distance from us. As the planet emerges into the evening sky and begins to become visible, this is the best time to see it. By the time the planet reaches greatest elongation, it has faded from anywhere between about -0.5 to +0.5. After greatest elongation, the planet will be visible for only a few more days because as it falls back into the solar glare it fades dramatically, by more than a tenth of a magnitude per day until it can no longer be picked out of the twilight glow.

Venus' phases are very regular. The planet reaches dichotomy within a very short range of time centered on greatest elongation. Mercury can be very irregular. The planet is always half-illuminated when it is at the bend in a right angle between the Sun, itself and Earth. But this is not always the point of greatest elongation. If its orbit were a perfect circle, as Venus' nearly is, then it would be so. But the eccentricity of Mercury's orbit causes some unusual things to happen. If Mercury is out at aphelion well before reaching that right-angle position then greatest elongation will come early just because it is several million miles farther from the Sun then than it is at the geometric position where greatest elongation should appear. Thus when Mercury is at greatest elongation in such instances it may be as much as 60% illuminated and thus brighter. When the reverse is true, with the planet at perihelion about two or three weeks before greatest elongation, the planet will pass through dichotomy and continue to pull away from the Sun in our sky because it really is pulling farther away from the Sun. So in this case, greatest elongation may occur a week after passing the right angle position. The planet may be barely 40% illuminated at greatest elongation as a result.

Because Mercury's synodic period is so short, it presents us with plenty of opportunities to see it each year. The planet will typically reach greatest

Figure 7.2. Mercury at greatest elongation and at dichotomy. Power Point illustration by author.

Figure 7.2. Mercury at greatest elongation and at dichotomy. Power Point illustration by author.

Figure 7.3. Mercury at sunset at its best and its worst. Power Point illustration by author.

elongation six times each year, three times to the east of the Sun (evening sky) and three times west of the Sun (morning sky). Typically in a given year in the evening sky, Mercury will give us one good apparition in the late winter or early spring, one very poor apparition in the late summer or early autumn and then one apparition that will occur around one of the solstices which will be somewhere in between the two. In the morning sky, Mercury will have a good apparition in late summer or early fall, a very poor one in late winter or early spring and then one apparition around the winter or summer solstice that will also be mid-level in viewing quality. At its best, Mercury may be in the sky for as long as 90-100 minutes after sunset. At an elongation that occurs around the solstice, Mercury will be in the sky for around 60-80 minutes after sunset and at its worst, even though it may be 28 degrees from the Sun, will not stay in the sky for more than 35-50 minutes after sunset. Four of the six apparitions give us a good chance to see Mercury. Remember that the best time for viewing Mercury, unlike Venus, is before greatest elongation when it is at its brightest, not later when the planet's disk may be larger but it will be fainter against a bright sky.

There is one other time for viewing Mercury that many people do not think of and that is during the day. Mercury can shine as bright as the brightest stars in the sky, so it is actually very easy to find in daylight. The problem is of course scanning for it in an unfiltered telescope in close proximity to the Sun. There are two easy and safe ways to view Mercury in daylight. The first and simpler way is to sight the planet in the morning sky before sunrise and continue to follow it away from the horizon as morning progresses. During the first two hours after sunrise, Mercury can climb as much as 35-40 degrees above the horizon during a favorable apparition. When Mercury is trailing the Sun during an evening apparition, install your solar filter and sight on the Sun. Then noting the difference between the right ascension of the Sun and Mercury, turn off the drive on your telescope for a period of time exactly equal to the difference in right ascension. If Mercury happens to be at exactly the same declination as the Sun, when the time is up and you turn the drive back on, the planet will be exactly in the center of your field of view after you remove the solar filter. If it is not, then you may have to use the setting circles of your telescope to move it north or south by a distance equal to the difference in declination difference between the two bodies.

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