Mercury is never more than twenty-eight degrees from the sun, and is
brightest when the distance between them is somewhere near twenty-two
degrees, or about four times the distance between the pointers in the
Big Dipper. The direction in which to search for him must always be
along the line of the ecliptic obliquely above the sun. Since his orbit
is inclined seven degrees to the ecliptic, he will be some place within
seven degrees of this line, on one side or the other. Within this
narrow strip in the sky, fourteen degrees wide and twenty-eight degrees
long, Mercury will be found whenever he is visible at all. And this
strip may be further shortened by at least twelve degrees; for when the
planet is nearer than that to the sun it is futile to attempt to see
him with the naked eye, save in very exceptional conditions. The five
degrees between the pointers will serve as an aid in measuring these
distances.
We can never see Mercury with the naked eye except when he is near one
elongation or the other; and even then he is visible only about an hour
after the sun is down in the evening or about an hour before it rises
in the morning. Three times each year he appears in the evening for
more or less than a week, according to the situation of the observer,
and three times a year he is visible in the morning for about the same
length of time. But, owing to his position with relation to us, the
evening exhibit that comes in the spring is the most favorable one for
a good view of him, and the morning appearance that is most favorable
is the one that comes in the autumn.
The mean synodic period of Mercury is about one hundred and sixteen
days, or a little less than four months. That is, he returns to
greatest eastern elongation and can be seen in the evening sky about
every one hundred and sixteen days, and the same length of time
elapses between his appearances in the morning sky at greatest western
elongation. But this mean synodic period is made up of synodic periods
varying in different revolutions from one hundred and five to one
hundred and thirty-four days. So, though one may mark the dates at
which the various positions of the planet occurred during any one
revolution, one cannot so easily determine the exact time at which
he will be found in the same positions at the next revolution; that
is, whether the revolution will take place in less or more than one
hundred and sixteen days. The earth and the planet are each traveling
at varying rates of speed, according as they are near the sun or
farther from it, and obviously it is a situation that requires careful
mathematical work to compute. The almanac must be referred to for the
exact date.
But, lacking an almanac, one will generally find that Mercury will
return to the same position relative to the earth and the sun within a
few days of his mean synodic period. Three periods, however much they
may vary individually, are almost always equal to three hundred and
forty-eight days, or three times the mean period. This is seventeen
days less than a year. Hence, if one is lucky enough to have seen
Mercury at eastern elongation one spring, and will look the next year
about seventeen days earlier, the planet will be found a little to
the east (about fifteen degrees) of where he was when first seen the
year before. He is there in the same position with relation to us and
the sun that he had the preceding spring, but in a slightly different
relation to us and the stars, because the sun lacks seventeen days of
having completed its apparent yearly journey around the zodiac. It must
still go through about one half of a constellation.
When Mercury shows himself at eastern elongation, he may be seen in the
west as an evening star for somewhere near a week, each evening drawing
nearer to the sun. When he disappears from view he passes between us
and the sun, and about four weeks later appears in the morning sky
before the sun rises. Under favorable conditions he is again visible
for a week or more; and then, again approaching the sun, he can be
seen no more for about ten weeks, during which time he passes through
superior conjunction on the other side of the sun from us and comes
back to eastern elongation.
Thus we can get, under very favorable conditions, six short views of
Mercury during the year--three in the evening and three in the morning.
So many views, however, are rarely secured by any but the professional
observer. The circumstances may well be considered felicitous if one
succeeds in getting a glimpse of him once or twice a year--at his
favorable situation in the evening in the spring and the morning
in the autumn. The sight of him, though, is truly worth a little
inconvenience--even to the extent of facing a cold evening wind in the
very early spring or getting out of a comfortable bed before dawn
during the first cool mornings of autumn.
It is hardly possible to say exactly where one can find Mercury at all
times during a long succession of revolutions. Moreover, it is not
necessary. These computations are made anew each year by experts in the
employ of the government, and the result is published in the _Nautical
Almanac_. From there it finds its way into all almanacs, so it is easy
of access to any one.
In the almanacs Mercury is represented by the sign (☿). It is a
conventionalized form of the caduceus, or wand, carried by the god
Mercury as a symbol of his power.
The next seven eastern and western elongations of Mercury occurring
after the publication of this book are as follows:
Eastern Elongation Western Elongation
(Evening Star). (Morning Star).
18 November, 1912. 27 December, 1912.
10 March, 1913. 24 April, 1913.
(Favorable for viewing.)
7 July, 1913. 22 August, 1913.
(Favorable for viewing.)
1 November, 1913. 10 December, 1913.
22 February, 1914. 6 April, 1914.
(Favorable for viewing.)
18 June, 1914. 5 August, 1914.
(Favorable for viewing.)
15 October, 1914. 23 November, 1914.