Space Science

What space science studies

Space science, also called astronomy, is the study of everything beyond the Earth’s air: the Moon, the Sun, the planets, the stars, and the universe as a whole.

Day, night, and the year

The Earth spins on its axis, an imaginary line through the North and South Poles. One full spin takes about 24 hours. The side facing the Sun has day; the side facing away has night.

The Earth also travels around the Sun. One full trip, or orbit, takes about 365 days: one year. The Sun’s gravity holds the Earth in its orbit.

Why we have seasons

Many people believe the seasons come from the Earth moving closer to the Sun and farther away. That is not the reason. In fact, the Earth is a little closer to the Sun in January than in July.

The seasons come from the tilt of the Earth’s axis. For part of the year, the Northern Hemisphere, where New York is, leans toward the Sun. Sunlight hits it more directly, and the days are longer: summer. Six months later, the Northern Hemisphere leans away from the Sun. Sunlight hits at a low slant, and the days are shorter: winter. When it is summer in New York, it is winter in Argentina and Australia.

The Moon

The Moon orbits the Earth about once a month. The Moon makes no light of its own; we see sunlight reflecting off it. As the Moon moves around the Earth, we see different amounts of its sunlit half. These changing shapes, from new moon to full moon and back, are the phases of the Moon.

The Moon’s gravity pulls on the Earth’s oceans, raising the water on the sides of the Earth nearest to and farthest from the Moon. As the Earth spins, most coasts pass through two high tides and two low tides about every day.

An eclipse happens when one body blocks the light to another. In a solar eclipse, the Moon passes between the Sun and the Earth and blocks the Sun. In a lunar eclipse, the Earth passes between the Sun and the Moon, and the Earth’s shadow falls on the Moon.

The solar system

The solar system is the Sun and everything held in orbit around it by its gravity. There are eight planets. The four closest to the Sun, Mercury, Venus, Earth, and Mars, are small and rocky. The four farther out, Jupiter, Saturn, Uranus, and Neptune, are giants made mostly of gas and ice. Moons, dwarf planets such as Pluto, asteroids, and comets also orbit the Sun.

Stars, galaxies, and the universe

The Sun is a star: a huge ball of hot gas that gives off light and heat. Deep in its core, hydrogen atoms are pressed together to make helium, and this process, called fusion, releases the energy we feel as sunlight. Stars are born from clouds of gas and dust, shine for millions or billions of years, and then die. Heavier elements, including the carbon and oxygen in our bodies, were made inside stars.

Stars are grouped in vast collections called galaxies. The Sun is one of billions of stars in our galaxy, the Milky Way, and there are billions of galaxies. Distances in space are so large that they are measured in light-years: the distance light travels in one year, about six trillion miles.

Scientists have found that the universe began about 13.8 billion years ago in a hot, dense state and has been expanding ever since. This explanation is called the Big Bang theory. One piece of evidence for it is that distant galaxies are moving away from us, and the farther away they are, the faster they move.