Astronomy Exam 1

Astronomical Unit

The average distance between Earth and the Sun, approximately 150 million kilometers (93 million miles). It is commonly used to measure distances within the solar system.

Light Year

The distance light travels through a vacuum in one year, approximately 9.46 trillion kilometers (5.88 trillion miles). It is used to measure distances to stars and galaxies.

Constellation

An officially recognized region of the sky, often associated with a recognizable pattern of stars. There are 88 officially recognized constellations.

Asterism

A recognizable pattern of stars that is not necessarily an official constellation. For example, the Big Dipper is an asterism within Ursa Major.

Apparent Magnitude

A measure of how bright a celestial object appears from Earth. Lower or more negative values indicate brighter objects.

Celestial Sphere

An imaginary sphere surrounding Earth on which celestial objects appear to be located. Astronomers use it as a model to describe positions in the sky.

Celestial Pole

Either of the two points where Earth's rotational axis, extended into space, intersects the celestial sphere.

Polaris

Also known as the North Star, Polaris lies very close to the north celestial pole and appears nearly stationary in the sky as Earth rotates.

Celestial Equator

An imaginary circle in the sky directly above Earth's equator, dividing the celestial sphere into northern and southern hemispheres.

Zenith

The point in the sky directly overhead an observer. The opposite point, directly beneath the observer, is called the nadir.

Right Ascension

A coordinate used to measure an object's position eastward along the celestial equator. It is usually measured in hours, minutes, and seconds.

Declination

A coordinate that measures how far north or south a celestial object lies from the celestial equator, expressed in degrees.

Circumpolar Constellations

Constellations that never set below the horizon for a particular observer because of their position relative to the celestial pole. Which constellations are circumpolar depends on the observer's latitude.

Equinox

One of the two times each year when the Sun crosses the celestial equator. Day and night are approximately equal in length around the world.

Solstice

One of the two times each year when the Sun reaches its greatest angular distance north or south of the celestial equator. These mark the longest and shortest daylight periods of the year.

Terminator

The boundary separating the illuminated side of a celestial body from its dark side. On Earth, it separates day from night.

Zodiac

A band of the sky along the ecliptic through which the Sun, Moon, and planets appear to move. It is divided into 12 traditional zodiac signs in Western astrology.

Earthshine

Faint light illuminating the dark portion of the Moon, caused by sunlight reflected from Earth onto the lunar surface and back toward us.

Ecliptic Plane

The plane of Earth's orbit around the Sun. The ecliptic is the apparent path of the Sun across the celestial sphere over a year.

Synchronous Orbit

An orbit in which an object's orbital period matches the rotation period of the body it orbits. A circular synchronous orbit around Earth is called geosynchronous; if it is also equatorial and prograde, it is geostationary.

Heliocentric

Describes a model or viewpoint that places the Sun at the center of a system.

Geocentric

Describes a model or viewpoint that places Earth at the center of a system.

Retrograde Motion

The apparent backward movement of a planet against the background stars, as seen from Earth. It occurs because Earth and the other planets move around the Sun at different speeds.

Ellipse

An oval-shaped closed curve. Many astronomical objects, including planets, travel in elliptical orbits around the Sun.

Focus

One of two fixed points inside an ellipse that determine its shape.

Eccentricity

A numerical measure of how stretched an ellipse is. An eccentricity of 0 describes a circle, while values closer to 1 describe increasingly elongated ellipses.

Spring Tide

A tide with an especially large difference between high and low water levels. It occurs near the new and full Moon, when the gravitational effects of the Sun and Moon reinforce one another.

Neap Tide

A tide with a relatively small difference between high and low water levels. It occurs near the first-quarter and third-quarter Moon, when the Sun and Moon pull at roughly right angles relative to Earth.

Kepler's 1st Law of Ellipses

Planets orbit the Sun in elliptical (oval-shaped) paths, with the Sun at one focus of the ellipse.

Kepler's 2nd Law of Equal Areas

A line connecting a planet to the Sun sweeps out equal areas in equal amounts of time. This means planets move faster when they are closer to the Sun and slower when they are farther away.

Kepler's 3rd Law of Harmonies

The square of a planet's orbital period is proportional to the cube of its average distance from the Sun. In other words, planets farther from the Sun take longer to complete an orbit.

Newton's 1st Law

An object at rest stays at rest, and an object in motion stays in motion at a constant velocity unless acted upon by an unbalanced external force.

Newton's 2nd Law

An object's acceleration depends on the net force acting on it and its mass. More force causes greater acceleration, while more mass causes less acceleration. Formula: F = ma.

Newton's 3rd Law

For every action force, there is an equal and opposite reaction force. When one object pushes on another, the second object pushes back with equal force in the opposite direction.

Newton's Law of Universal Gravitation

Every object with mass attracts every other object with mass. The gravitational force increases with the masses of the objects and decreases as the square of the distance between them increases.

Ptolemy

Developed the geocentric model, in which Earth is at the center of the universe and the Sun, planets, and stars orbit Earth.

Copernicus

Proposed the heliocentric model, placing the Sun at the center of the planetary system, with Earth rotating on its axis and orbiting the Sun. Calculated circumference of Earth to high accuracy.

Tycho Brahe

Made extremely precise astronomical observations. Proposed a hybrid model in which the Sun orbits Earth while the other planets orbit the Sun.

Kepler

Used Tycho's observations to develop three laws of planetary motion, showing that planets orbit the Sun in ellipses rather than perfect circles.

Galileo

Used a telescope to observe Jupiter's moons, the phases of Venus, and features on the Moon. His discoveries supported the heliocentric model and challenged the idea that everything orbits Earth.

Newton

Developed the three laws of motion and the law of universal gravitation, explaining why planets orbit the Sun and providing a physical basis for Kepler's laws.

Solar Eclipse

Occurs when the Moon passes directly between the Sun and Earth causing the Moon to cast it’s shadow on Earth, blocking the Sun's light.

Lunar Eclipse

Occurs when the Earth passes directly between the Sun and the Moon, causing the Earth's shadow to fall on the Moon.