MERCURY AND VENUS.
Now let us take a flight through the Sun’s great kingdom, paying a
visit to one bright world after another on our way. We will start from
near the Sun himself, stopping first to look at the two innermost of
the Four Lesser Planets. We might name them The Sun’s Body-Guard.
MERCURY has a pathway round the Sun much more oval in its shape than
our Earth’s pathway. And the Sun is a good deal to one side of the
exact middle of that oval. So Mercury, at one time of his year, is many
millions of miles closer to the Sun than at another time.
You must remember that a Planet’s Year means just the length of time
that the Planet takes to go once round the Sun. Our Earth’s yearly
journey takes 365 days; but other worlds have years either longer or
shorter. No two are exactly alike.
The length of Mercury’s year is about 88 of our days, or three months
of Earth-time. So four years of Mercury go to one of our years. If a
little boy on Mercury had lived just as long as ten of our years, he
would be forty years old!
When closest to the Sun, Mercury cannot easily be seen by us, he is so
lost in the Sun’s radiance. And although perhaps the brightest of the
worlds, because so much the nearest, he often seems dim to us.
At his farthest off point from the Sun we have our best view of him,
because he then stays longer above the horizon after the sunlight sinks
away.
We never see Mercury high up in the sky, for when Mercury is high the
Sun is up also; and when the Sun can be seen Mercury cannot be seen
without a telescope.
The distance of Mercury from the Sun is commonly said to be about
thirty-six millions of miles. That may be called his “middle-distance.”
He draws sometimes as near to the Sun as twenty-eight millions of
miles, and goes as far off as forty-three millions of miles--a very
great difference.
Even at his farthest Mercury has to endure an awful blaze of heat and
glare; and at his nearest the most scorching mid-day ever known on the
hottest parts of the Earth would be icy by comparison.
Mercury does not always go at the same pace through the sky. When near
he travels faster, because the Sun’s pull is stronger. When farther off
he slackens his speed.
At his quickest he whirls onward at the rate of about _thirty-five
miles each second_! Think of that! A railway train does pretty well if
it gets over about thirty-five miles of ground each _hour_, and sixty
or seventy miles an hour we count very fast travelling. But Mercury’s
speed is more than two thousand miles an hour. This quite puts our
express trains to the blush.
It is impossible for us to see much of a planet so bathed in sunlight
glory.
We do not know whether the axis of Mercury does or does not slant, like
the Earth’s axis. Nor are we at all sure how long it takes Mercury to
spin upon his axis.
Wonderful as it sounds, Mercury has been weighed by man--and not only
Mercury, but the Moon, and the Sun, and the other planets, and even
some of the Stars. I cannot try, in a book such as this, to explain
_how_ the heavenly bodies are weighed from our little Earth. I can only
tell you that it is really and truly done.
Mercury is a far heavier body than our Earth; not actually more heavy
as a whole, because so much smaller, but heavier in _make_.
Do you see what this means? Iron is heavier than tin in its make. A
large quantity of tin may weigh more than a small lump of iron; yet in
actual make the iron is heavier. If Mercury were as big as our Earth,
Mercury would be very much the heavier of the two. Our Earth is less
heavy in make than Mercury, but she comes next after Mercury. Other
worlds are still lighter.
Mercury shines less brightly than Venus, as seen from the Earth. We
have to allow for the greater distance of Mercury; but even then Venus
seems to be more brilliant than one would expect, while Mercury is less
brilliant.
Although the Sun pours his beams upon all things alike, those beams are
not always received alike. Some worlds make more of the light which
they have than do others, and they give out more shining in return.
We see that even on the Earth. If a sheet of polished silver and a
sheet of unpolished lead are held side by side in the sunlight, what
a difference we find! The silver flashes brilliantly, while the lead
shows only a dull sort of brightness.
Of these two worlds, so near to the Sun, Mercury, the nearer, is said
to shine only like lead, while Venus, the farther, shines like silver.
Once in a while the tiny body of Mercury is seen to creep, as a little
black dot, across the face of the Sun: though this is only visible in a
telescope. Then we have a “Transit of Mercury.” In the last chapter you
were told what is meant by a Transit.
If Mercury were as near to us as our Moon is, he would hide the Sun
from us in his transit just as the Moon does in an eclipse--only more
fully, because Mercury is bigger than our Moon.
In a Transit, as in an Eclipse, there is no real drawing together of
Sun and Planet. Be very clear in your mind about this. Mercury glides
_between_ our Earth and the Sun, but he is just as far as usual from
the Earth on one side and from the Sun on the other side.
If you are gazing at a church-tower a great many miles away, and a bird
near at hand flies between, hiding for a moment that tower from you,
the bird may be said to “eclipse” the tower. But he does not go nearer
to the church. He only moves into the straight line between you and the
tower.
And if, instead of this, a bird some distance off flies between--then
you have a “transit.” The more distant bird cannot hide the
church-tower, but you see his little body pass across it, as a dark
spot.
A transit of Mercury is not common. For though Mercury often passes
between the Earth and the Sun he is not often _exactly_ between. His
oval pathway is not quite on the same level as the Earth’s pathway. So
he is usually a little too high or a little too low for us to see him
against the Sun.
Leaving Mercury behind we come next to the pathway of the planet VENUS.
Mercury was a good deal smaller than our Earth; but Venus is almost
the same size. Instead of being, like Mercury, only some thirty-six
millions of miles away from the Sun, Venus is about sixty-five millions
of miles off. She journeys round him at a rate of some twenty-two
miles each second, and her year lasts about seven months and a half of
Earth-time. In make she is not quite so solid and heavy as our Earth.
Venus in her journey round the Sun, as our Moon travels round the
Earth, is now believed to turn on her axis so very slowly that the same
side is always towards the Sun, and the other side is always turned
away. If this really is so, one half of Venus has an endless day, and
the other half an endless night.
The same state of things may possibly be also true of Mercury.
Both these worlds are believed to have air around them, and Venus
seems to be enwrapped in thick clouds. This helps to explain the great
brilliancy of Venus. Nothing lights up so well in sunshine as masses
of cloud, though of course we on Earth more generally see the dark
_undersides_ of clouds.
So far as we know, Venus is a lonely world. She seems to have no
little moon-friend to journey with her in the sky.
She has, however, a far more splendid Sun than ours--the very same Sun
only much nearer, and bigger and more dazzling. She also has in her sky
a very exquisite little shining Earth, far lovelier than Venus at her
best ever appears to us. And I will tell you why.
Venus comes at times nearer to our Earth than any other world in all
the sky, except our Moon. If the Moon is our little Sister-World Venus
is our Next-Door Neighbor.
When the Earth happens to be on one side of the Sun and Venus on the
other side the two then are widely parted. When both are on the same
side of the Sun at once they are quite near--divided by only about
twenty-six millions of miles.
Of course twenty-six millions of miles sound a good deal to you and
me. We think so much of even one thousand miles on the Earth, and
one million is a thousand thousand. But in talking of sky-distances
twenty-six millions of miles are merely a matter of next-door neighbors!
Unfortunately, when Venus is at her nearest to us we cannot see her.
She is then, like our Moon at New-Moon, between us and the Sun, so that
her dark side is toward us, and her bright side is away from us.
This is a great pity, because she would be a lovely sight then, so near
and brilliant. Our best sight of her is when she is away to one side,
and then it is really only “Half-Venus” that we see. Even that half is
the brightest of all heavenly bodies to us, after the Sun and Moon; but
you can fancy how much more beautiful the whole would be.
We do see the whole of her when she gets right beyond the Sun; but then
she is so very, very far away that she becomes much more small and dim.
However, when Venus is New-Venus to us--like the Moon being
New-Moon--our Earth is Full-Earth to Venus. Then indeed our Earth must
be a splendid sight, if only there were anybody on Venus to admire her!
When Venus comes between us and the Sun she is more commonly not
_exactly_ between. Now and then, however, instead of being a little
higher or lower, she is just precisely between, and so we have a
Transit of Venus. It is much the same as a Transit of Mercury. Only the
round black dot is bigger, and can be seen more easily; sometimes even
without a telescope.
Two transits of Venus come near together, within a few years. Then for
more than a hundred years there is no transit; after which two more
come again.
Venus can never see a transit or passing of our Earth over the Sun,
because the pathway of the Earth lies outside the pathway of Venus. So
our Earth can never pass between Venus and the Sun.
But Venus can see a transit of Mercury; and we on Earth can see
transits of Mercury and Venus. And Mars doubtless can see transits of
Mercury, Venus and Earth, though the Earth can never see a transit of
Mars.
It is always an _outer_ planet which sees an _inner_ planet seem to
pass across the Sun’s face.
In all these cases, if the worlds were very near together--as near as
our Moon is to the Earth--the Transits would be Eclipses.
QUESTIONS.
1. How far is Mercury from the Sun?
Sometimes nearer, sometimes farther; but, roughly, about 36 millions of
miles.
2. How fast does Mercury journey?
At his fastest, about 35 miles each second.
3. How long is Mercury’s year?
About 88 days, or three months, of Earth-time.
4. Can we see much of Mercury?
No; because it is too near to the Sun.
5. Is Mercury heavy or light in make?
Much heavier in make than our Earth is.
6. Which is brighter, Mercury or Venus?
Mercury gets most sunlight, but Venus reflects sunlight best.
7. What is a Transit of Mercury or Venus?
The planet passes exactly between Earth and Sun, and is seen against
the Sun, crossing his face.
8. What distance is Venus from the Sun?
About 66 millions of miles.
9. How fast does Venus journey?
About 22 miles each second.
10. Why is Venus slower than Mercury?
Because Venus is farther off than Mercury from the Sun, and so the pull
of the Sun is less.
11. How long is the year of Venus?
About seven months and a half of Earth-time.
12. Is any other planet in our sky brighter than Venus?
No planet or star--only the Sun and the Moon.
13. How near to us does Venus come?
At her nearest she is about 26 millions of miles off.
14. Is she very bright then?
Her bright side is turned away from us then, and we cannot see her at
all.
15. When is our best view of Venus?
When we see her as really Half-Venus.