A Backyard Glossary
Short, plain-language definitions for the terms that come up constantly in observing plans, ephemerides, and basic optics — written the way they're actually used, not the way a textbook would phrase them.
A nine-level scale describing how much a location's sky glow drowns out fainter stars, from Class 1 (a true dark-sky site) to Class 9 (an inner-city sky). Most suburban yards sit around Class 6 or 7.
How steady the atmosphere is on a given night. Poor seeing makes stars shimmer and blurs fine planetary detail, even under an otherwise clear sky.
How clear the air itself is — haze, humidity, and thin cloud all lower transparency, dimming faint objects even when seeing is steady.
Looking slightly to the side of a faint object rather than straight at it, which shifts the image onto a part of the eye more sensitive to dim light.
The gradual process by which eyes become more sensitive to low light after time away from bright screens or porch lights — typically twenty to thirty minutes for a useful gain.
The alignment of a telescope's mirrors or lenses along a single optical axis. A reflector that's fallen out of collimation produces soft, distorted views even with clean optics.
The width of the beam of light leaving an eyepiece, found by dividing aperture by magnification. If it's wider than your eye's pupil at night, that extra light is wasted.
How much sky is visible at once through an eyepiece or pair of binoculars, usually given in degrees — a wider field makes it easier to find and track a target.
The line dividing the lit and shadowed halves of the Moon. Craters and ridges cast their longest, sharpest shadows right along this edge, which is why it's a favorite spot to observe.
The point when a planet sits opposite the Sun in the sky, rising as the Sun sets. It's usually the point in a planet's cycle when it's largest and brightest as seen from Earth.
The moment two objects — often two planets, or a planet and the Moon — appear closest together in the sky from Earth's point of view.
A scale for how bright an object appears. Lower numbers mean brighter; each step of about 5 corresponds to roughly a hundredfold change in brightness.
The point directly overhead. Objects near the zenith are usually seen through the least atmosphere and light pollution, which is often the clearest part of a lit sky.
The moment an object reaches its highest point above the horizon for the night, usually the best window to observe it before it starts sinking again.
A simple telescope mount that moves up-down (altitude) and side-to-side (azimuth), the same way a camera tripod head works — intuitive, though it requires manual tracking.
Finding a faint target by navigating from a brighter, easy-to-spot star through a known pattern of dimmer stars, rather than relying on go-to electronics.
A table of predicted positions for a celestial object over time — where the Moon or a planet will be, and when, on a given date and location.
The faintest magnitude a given sky allows an unaided eye to see. Under heavy light pollution, that limit can sit several magnitudes brighter than under a dark rural sky.
A telescope's or instrument's first use after being set up or assembled — often treated as a small milestone, and a useful moment to note collimation and focus quirks.
The glow that hangs over a town or city, brightest near the horizon in that direction, that can wash out low-altitude objects long before overhead ones are affected.