The sun comes out in the morning and sets at night, and then the moon rises. Right? Then how come we can see the moon during the day sometimes?

The moon and the sun don’t actually take turns in the sky. The earth goes around the sun, and the moon goes around the earth. All three are spinning (旋转) in space all the time.

The moon moves through the sky night and day. Much of the time, the sunlight is so bright that we can’t see the moon during the day.

The sun is a very hot fire ball, which gives off both heat and light. But the moon is made of rock, it has no fire, and it is not hot or bright at all. What we call moonlight is actually sunlight bouncing off the moon, the same way that sunlight bounces off the glass in the window. They look like they’re lighting up, but they’re really just passing the sunlight along.

How bright the moon is depends on where it is in the sky. If the moon is between the earth and the sun, it doesn’t bounce any sunlight to us at all, and it is very hard to see, even at night. This is called a new moon.

When the moon is behind the earth, it shines very brightly. We call this a full moon. Sometimes, when the moon is very bright, we can see it even during the daytime.

When it comes to long-distance space travel, the biggest problem is that spaceships can only get as far as the fuel onboard allows. In 1964, a British writer Clarke came up with the idea of “solar sailing” in his science fiction. He wrote of large, mirror-like sails that collect power from the sun and free spaceships from the limitation of fuel.

This sci-fi idea was first successful brought to reality in 2010 by Japan’s IKAROS satellite. Ever since then, the technology has improved mission by mission, with the latest being NASA’s Advanced Composite Solar Sail 3(ACS3). NASA has recently announced that it’s going to team up with Lithuania’s satellite-building company NanoAvionics to test its new sail in Low-Earth Orbit.

To many, a solar sail may look like a giant solar panel, but it actually works very differently. While solar panels take in energy from the sun and change it into power, solar sails use Photons given off by the sun to create direct power. Besides, solar panels are big and heavy to carry, while solar sails are just the opposite.

According to Abbott, the CEO of NanoAvionics, the upcoming Low-Earth Orbit trial with NASA, if successful, will be followed with deep-space missions that involve larger solar sails. These are ideal for low-cost and long-distance space travel because fuel will no longer be a problem.

“It started as a sci-fi dream,” Popular Mechanics reporter Jennifer Leman wrote. “Now it could be the future of space flight.”

Have you ever dreamed of traveling in space? To travel in space, we first have to work out where our food will come from.

Today's space travellers don't go to the Moon or Mars. They go to the International Space Station（ISS）. There, the spacemen live for weeks to months.

If you visited the ISS years ago, nearly every kind of food you ate would have come from the Earth.

In 2014, scientists sent the ISS a space garden with some vegetable seeds（种子）. With water and some light, the vegetables in the garden grew. Those were the first foods being grown on the ISS. But the spacemen couldn't eat them and had to send them back to the Earth. The next year, after scientists made sure they were safe, the spacemen grew a second crop. This time the vegetables were ready for the spacemen to eat.

Gardening is different in space than it is on Earth. Without gravity, plants don't know which way is up. But they learn how to grow. They send their leaves in the direction of light and their roots into the soil.

The spacemen also have grown vegetables as well as flowers. It's important to know a plant can flower in space, because flowering is part of how some plants make fruit.

While the space garden is small for now, hopefully it could one day help provide people with food on long-time space trips.