Introduction to astronomy
Astronomy is the study of the Sun, Moon, planets, stars and the universe. People have studied the sky for thousands of years to tell time, plan farming and fishing, and to travel. The universe is everything that exists. It contains billions of galaxies, each with billions of stars. Our galaxy is the Milky Way. Distances are so large that astronomers use the light year, the distance light travels in a year (about 9.5 × 10¹² km), and the astronomical unit (AU), the average distance from the Earth to the Sun (150 million km).
The solar system
The solar system is the Sun and everything that orbits it. The eight planets, in order from the Sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four inner planets are small and rocky; the four outer planets are large and made mostly of gas and ice. Between Mars and Jupiter is the asteroid belt. Comets are balls of ice and dust that develop long tails near the Sun.
| Planet | Distance from Sun (AU) | Orbital period (years) | Moons |
|---|---|---|---|
| Mercury | 0.39 | 0.24 | 0 |
| Venus | 0.72 | 0.62 | 0 |
| Earth | 1.00 | 1.00 | 1 |
| Mars | 1.52 | 1.88 | 2 |
| Jupiter | 5.20 | 11.9 | 95 |
| Saturn | 9.54 | 29.5 | 146 |
| Uranus | 19.2 | 84 | 28 |
| Neptune | 30.1 | 165 | 16 |
Choose the distance of a planet from the Sun in astronomical units. Find the length of its year.
The Earth, Moon and Sun
The Earth rotates on its axis once in 24 hours, giving day and night. It revolves round the Sun once in a year (365¼ days). Its axis is tilted by 23.5°, which causes the seasons: when the southern hemisphere leans towards the Sun it has summer. In Tanzania, near the equator, the day length changes little and the seasons are described as wet and dry.
The Moon orbits the Earth once about every 29.5 days. Its phases are caused by how much of its sunlit half we can see. An eclipse of the Sun happens when the Moon passes between the Sun and the Earth and its shadow falls on the Earth. An eclipse of the Moon happens when the Earth lies between the Sun and the Moon and the Moon enters the Earth’s shadow.
Constellations
A constellation is a group of stars that seems to form a pattern in the sky. The stars in a constellation are not near each other in space; they only look close from Earth. Well-known constellations include the Southern Cross (Crux), used to find south in the southern hemisphere, Orion (the hunter), Scorpius, Ursa Major (the Plough) and Leo. The pattern appears to move across the sky because the Earth rotates. Farmers and sailors in East Africa used constellations to know the planting seasons and to navigate on the Indian Ocean.
Key terms
- Light year
- The distance light travels in one year.
- Constellation
- A pattern of stars seen from Earth.
- Galaxy
- A huge group of stars, gas and dust.
- Eclipse
- The blocking of light from the Sun or Moon by another body.
Summary
- The universe contains galaxies of stars; distances are measured in light years and AU.
- The solar system has eight planets, with inner rocky and outer gas giants; T² ∝ r³.
- Rotation gives day and night; the tilted axis and revolution give seasons; the Moon causes phases and eclipses.
- Constellations are patterns of stars used for navigation and calendars.