Astronomy is the scientific study of celestial objects, space, and the physical universe, while cosmology focuses specifically on the origin, evolution, and large-scale structure of the universe as a whole. To talk about the vast distances involved, astronomers rely on a few key units. One Astronomical Unit (AU) is the average distance from Earth to the Sun, about 150 million kilometers. A light-year, the distance light travels in one year, equals roughly 9.46 trillion kilometers, and sunlight takes about 8 minutes and 20 seconds to reach Earth. A parsec, equal to about 3.26 light-years, is defined as the distance at which 1 AU subtends an angle of one arcsecond and is widely used in professional astronomy.
At the center of our Solar System sits the Sun, a G-type main-sequence yellow dwarf star about 4.6 billion years old. Its energy comes from nuclear fusion of hydrogen into helium in its core, where the surface temperature is about 5,500 °C. Above the surface lies the corona, the Sun's outermost atmosphere, which is paradoxically far hotter than the surface itself. Solar activity includes sunspots (cooler, darker magnetic regions), sudden solar flares (intense bursts of radiation), and the constant outflow of charged particles known as the solar wind, which together define the heliosphere, the vast bubble of space dominated by the Sun's plasma and magnetic field.
The Solar System contains eight planets, since Pluto was reclassified as a dwarf planet in 2006. The inner rocky planets are Mercury, Venus, Earth, and Mars, while the outer planets divide into the gas giants, Jupiter and Saturn, and the ice giants, Uranus and Neptune. Beyond Neptune lies the Kuiper Belt, a disk-shaped region of icy bodies including Pluto, and even farther out the Oort Cloud forms a spherical shell of icy objects at the very edge of the Sun's gravitational influence. Between Mars and Jupiter orbits the asteroid belt, populated by small rocky bodies. Other small bodies include comets, such as the famous Halley's Comet that returns every 76 years, whose tails are produced when sunlight and solar wind push vaporizing gas and dust away. When small rocky or icy fragments enter Earth's atmosphere, they produce meteors (the streak of light), and any survivors that reach the ground are called meteorites.
Earth's seasons arise from its axial tilt of about 23.5°, while equinoxes mark days when day and night are nearly equal and solstices mark the longest and shortest days. The Moon, about 3,474 km in diameter, likely formed around 4.5 billion years ago from a giant impact between Earth and a Mars-sized body sometimes called Theia. The Moon's and Sun's gravitational pull produces tides on Earth: spring tides occur when the three bodies align, and neap tides when the Sun and Moon sit at right angles. When Earth's shadow falls on the Moon, a lunar eclipse occurs, while a solar eclipse happens when the Moon passes between the Sun and Earth. Depending on the Moon's distance, a total eclipse fully covers the Sun's disk, an annular eclipse leaves a bright ring, and a partial eclipse covers only part of the Sun. The familiar patterns of stars in our sky are organized into 88 official constellations, with the twelve zodiac constellations lying along the ecliptic, the Sun's apparent annual path.