Mercury is the smallest member of the sun's family. It is only 3.100 miles across. It is also the sun's swiftest planet. Its yearly journey round the sun is only 85 days.
Mercury always keeps one side towards the sun. On this side it is always day, on the other side always night. We only see the lighted side.
Mercury appears to us like a yellowish orange star. The nearest planet to the sun, it is always seen near the sun, either just before sunrise or soon after sunset. People sometimes call mercury the morning star or evening star.
Mercury is half the size of the earth. Because it is much lighter, it has much less gravity. If you can visit Mercury in a spaceship, you will find it a strange world. Its low gravity makes you feel very light. If your weight on earth is 100 pounds, your weight on Mercury is only 27 pounds.
Looking at the sun from Mercury, you can see that it's much more brilliant than it is seen from the earth. And the yellow centre of the sun appears three times bigger from Mercury.
On its lighted side, Mercury's temperature is about 300 degrees centigrate. But the dark side is extremely cold, but 150 degrees below zero so mercury is probably the coldest as well as the hottest of the planets.