Below is a list of terms that transcribers might come across in PHaEDRA notebooks. (Please feel free to send us suggestions!)

Aperture The diameter of a telescope’s main lens or mirror — and the scope’s most important attribute. As a rule of thumb, a telescope’s maximum useful magnification is 50 times its aperture in inches (or twice its aperture in millimeters).

Asterism Any prominent star pattern that isn’t a whole constellation, such as the Northern Cross or the Big Dipper.

Celestial Coordinates A grid system for locating things in the sky. It’s anchored to the celestial poles (directly above Earth’s north and south poles) and the celestial equator (directly above Earth’s equator). Declination and right ascension are the celestial equivalents of latitude and longitude.

Cepheid variable One of an important group of yellow giant or supergiant pulsating variables, named after the prototype, Delta Cephei. The significance of Cepheid variables became apparent when H. S. Leavitt discovered that their period was directly related to their absolute magnitude. The resulting period-luminosity relationship is used to determine distances.

Constellation A distinctive pattern of stars used informally to organize a part of the sky. There are 88 official constellations, which technically define sections of the sky rather than collections of specific stars.

Declination (Dec.) The celestial equivalent of latitude, denoting how far (in degrees) an object in the sky lies north or south of the celestial equator.

Double Star (Binary Star) Two stars that lie very close to, and are often orbiting, each other. Line-of-sight doubles are a consequence of perspective and aren’t physically related. Many stars are multiples (doubles, triples, or more) gravitationally bound together. Usually such stars orbit so closely that they appear as a single point of light even when viewed through professional telescopes.

Eclipse An event that occurs when the shadow of a planet or moon falls upon a second body. A solar eclipse occurs when the Moon’s shadow falls upon Earth, which we see as the Moon blocking the Sun. When Earth’s shadow falls upon the Moon, it causes a lunar eclipse.

Ecliptic The path among the stars traced by the Sun throughout the year. The Moon and planets never stray far from the ecliptic.

Lightcurve A graph of the variations in brightness of an object, particularly a variable star or a rotating asteroid, plotted against time.

Magnitude A number denoting the brightness of a star or other celestial object. The higher the magnitude, the fainter the object. For example, a 1st-magnitude star is 100 times brighter than a 6th-magnitude star.

Meridian The imaginary north-south line that passes directly overhead (through the zenith).

Messier object An entry in a catalog of 103 star clusters, nebulas, and galaxies compiled by French comet hunter Charles Messier (mess-YAY) between 1758 and 1782. The modern-day Messier catalog contains 109 objects.

Meteor A brief streak of light caused by a small piece of solid matter entering Earth’s atmosphere at tremendous speed (typically 20 to 40 miles per second). Also called a “shooting star.” If material survives the trip through the atmosphere, it’s called a meteorite after landing on Earth’s surface.

Nebula Latin for “cloud.” Bright nebulas are great clouds of glowing gas, lit up by stars inside or nearby. Dark nebulas are not lit up and are visible only because they block the light of stars behind them.

Reflector A telescope that gathers light with a mirror. The Newtonian reflector, designed by Isaac Newton, has a small second mirror mounted diagonally near the front of the tube to divert the light sideways and out to your eye.

Refractor A telescope that gathers light with a lens. The original design showed dramatic rainbows, or “false color,” around stars and planets. Most modern refractors are achromatic, meaning “free of false color,” but this design still shows thin violet fringes around the brightest objects. The finest refractors produced today are apochromatic, meaning “beyond achromatic.” They use expensive, exotic kinds of glass to reduce false color to nearly undetectable levels.

Right Ascension (R.A.) The celestial equivalent of longitude, denoting how far (in 15°-wide “hours”) an object lies east of the Sun’s location during the March equinox.

Spectral classification The categorization of stars according to the properties of their spectra. The first attempt to do so was the Secchi classification in the 1860s, but it was the Harvard classification scheme that led to the current system of spectral types. Stars were classified as type O, B, A, F, G, K, or M in order of decreasing surface temperature, and each type furthur subdivided into subclasses from 0 (hottest) to 9 (coolest).

Star A massive ball of gas that generates prodigious amounts of energy (including light) from nuclear fusion in its hot, dense core. The Sun is a star.

Star Cluster A collection of stars orbiting a common center of mass. Open clusters typically contain a few hundred stars and may be only 100 million years old or even less. Globular clusters may contain up to a million stars, and most are at least 10 billion years old (almost as old as the universe itself).

Variable Star A star whose brightness changes over the course of days, weeks, months, or years.


Dictionary sources:

“Glossary of Astronomy | Astronomy Terms & Names.” Sky & Telescope, Sky & Telescope Media, 2018,

Ridpath, Ian. A Dictionary of Astronomy (2nd Revised Ed.). Oxford University Press, 2012.