Astronomy reference data
Celestial objects, phenomena, and cosmology.
Topic guide
What Astronomy covers
Astronomy reference data covers what is in the sky and how humans learned about it: the planets and other bodies of the Solar System, the phases of the Moon that follow from its orbit, the types of stars and their life stages, the constellations used to map the sky, and the missions that carried instruments and people beyond Earth. Distances and periods are measured quantities that improve over time, so the numbers are the best-known values at a point in time, while the classifications (planet, dwarf planet, star type) are partly conventions set by the International Astronomical Union.
Key ideas
- Astronomical unit
- About 149.6 million kilometers: the mean distance from Earth to the Sun, used to measure distances inside the Solar System.
- Orbital period
- The time a body takes to complete one orbit. It grows with the orbit's size, as described by Kepler's third law.
- Dwarf planet
- A body orbiting the Sun that is round but has not cleared its orbital neighborhood. Pluto and Ceres are examples.
- Main sequence
- The long stable phase of a star's life when it fuses hydrogen into helium in its core.
- Synodic month
- The cycle of lunar phases, about 29.5 days, which is longer than the Moon's orbit relative to the stars.
- Light-year
- The distance light travels in one year, about 9.46 trillion kilometers; a unit of distance, not of time.
- Constellation
- A region of the sky, officially 88 of them, and traditionally a pattern of stars. The stars in one are usually at very different distances.
Common pitfalls
- Mixing astronomy with astrology. The zodiac signs and the astronomical constellations do not line up with the dates, because of precession and unequal constellation sizes.
- Treating moon counts and similar tallies as fixed. They change as new moons are confirmed.
- Reading a light-year as a unit of time, or assuming all stars in a constellation are close to each other.
- Quoting planetary distances as constants; orbits are ellipses, so distances vary through the year.
Using it from an agent
- Give a measured value with its unit and, when it changes, the date or source it comes from.
- Say whether a classification such as 'planet' is an IAU convention when the question is about Pluto or other borderline bodies.
- When asked about horoscopes or zodiac signs, answer the astronomical question accurately and label astrology as a separate, non-scientific tradition.
Start with: Solar System Planets · Types of Stars
Datasets in Astronomy
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Phases of the Moon
The primary phases of the lunar cycle.
8 entries -
Solar System Planets
The eight planets of the solar system.
10 entries -
Space Mission Milestones
Landmark missions in the history of space exploration.
25 entries -
Types of Stars
Major stages and classifications of stars.
13 entries -
Zodiac Constellations
The twelve constellations of the zodiac.
13 entries
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