For a very long time astronomers could not discover what time Venus took in rotating on her own axis--that is to say, what the length of her day was. She is difficult to observe, and in order to find out the rotation it is necessary to note some fixed object on the surface which turns round with the planet and comes back to the same place again, so that the time it takes in its journey can be measured. But the surface of Venus is always changing, so that it is impossible to judge at all certainly. Opinions differ greatly, some astronomers holding that Venus's day is not much longer than an earthly day, while others believe that the planet's day is equal to her year, just as in the case of Mercury. Venus's year is 225 days, or about seven and a half of our months, and if, indeed, her day and year are the same length, very peculiar effects would follow. For instance, terrible heat would be absorbed by the side of the planet facing the sun in the perpetual summer; and the cold which would be felt in the dreary winter's night would far exceed our bitterest Arctic climate. We cannot but fancy that any beings who might live on a planet of this kind must be different altogether from ourselves. Then, there is another point: even here on earth very strong winds are caused by the heating of the tropics; the hot air, being lighter than the cold air, rises, and the colder air from the poles rushes in to supply its place. This causes wind, but the winds which would be raised on Venus by the rush of air from the icy side of the planet to the hot one would be tornadoes such as we could but faintly dream of. It is, of course, useless to speculate when we know so little, but in a subject so intensely interesting we cannot help guessing a little. Venus is only slightly smaller than the earth, and her density is not very unlike ours; therefore the pull of gravity must be pretty much there what it is here--that is to say, things will weigh at her surface about the same as they do here. Her orbit is nearly a circle, so that her distance from the sun does not vary much, and the heat will not be much greater from this cause at one time of the year than another. As her orbit is tilted up a little she does not pass between us and the sun at each revolution, but occasionally she does so, and this passing is called a transit. Many important facts have been learned by watching these transits. Mercury also has transits across the sun, but as she is so much smaller than Venus they are not of such great importance. It was by the close observation of Venus during her transits that the distance from the earth to the sun was first measured. Not until the year 2004 will another transit of Venus occur.