| A | B |
|---|
| astrolabe | an instrument invented by the Ancient Arabs and used to measure an object’s altitude |
| astronomy | the study of all physical objects beyond Earth |
| calendar | a system for organizing time |
| year | the time required for the Earth to orbit the sun once |
| month | the time required for the moon to orbit the Earth once |
| day | the time required for the Earth to rotate once on its axis |
| leap year | a year in which an extra day is added to the calendar |
| Julian calendar | calendar that originally introduced the concept of leap year |
| Gregorian calendar | calendar that is currently used by most countries |
| seasonal cycles of the sun, moon, and stars | The ancient cultures based their calendars on these. |
| summer solstice | the longest day of the year |
| eclipse | an event that occurs due to the particular alignment of the sun, moon, and Earth |
| Aristotle | Greek philosopher who successfully explained the phases of the moon and eclipses and argued that the Earth is a sphere |
| Claudius Ptolemy | a Greek astronomer who in 140 AD described the Earth as the center of the universe resulting in the sun and other planets revolving around the Earth |
| Nicolaus Copernicus | Polish astronomer who in 1543 stated that the sun was the center of the universe resulting in the Earth and other planets revolving around the sun |
| Tycho Brahe | Danish astronomer who used large observation tools to observe the sky and believed that the planets revolved around the sun and the sun and moon revolved around the Earth |
| Johannes Kepler | Danish astronomer who in 1609 discovered that all the planets revolved around the sun in elliptical orbits and that the sun was not in the exact center of the orbits |
| Galileo Galilei | Italian astronomer who was the first to use a telescope to discover to discover that that planets are physical bodies and not just dots of light |
| Isaac Newton | the person who in 1687 explained that the planets orbit the sun and the moons orbit the planets due to gravity |
| William Herschel | astronomer who discover Uranus in 1781 and recorded sightings of small, fuzzy patches which were later identified as other galaxies |
| Edwin Hubble | person who used photography in 1923 to discover additional galaxies in the universe and that the universe is expanding |
