Your guide to understanding the common terms used on this site and in amateur astronomy.

Altitude & Azimuth

This is a coordinate system used to pinpoint objects in your local sky. Altitude is the object's height above the horizon, measured in degrees (0° is the horizon, 90° is directly overhead). Azimuth is the object's direction along the horizon, measured in degrees (0° is North, 90° is East, 180° is South, 270° is West).

Averted Vision

A crucial technique for viewing faint objects. Instead of staring directly at your target, you look slightly to the side. This uses the more light-sensitive peripheral cells in your retina, often making the faint object "pop" into view.

Culmination

The point at which a celestial object reaches its highest position in the sky for the night. This is generally the best time to observe it, as you are looking through the least amount of Earth's distorting atmosphere.

Light Pollution

The brightening of the night sky caused by artificial lights from cities and towns. This glow washes out faint celestial objects like galaxies and nebulae, making them difficult or impossible to see. Finding a dark site away from city lights is key to deep-sky observing.

Seeing vs. Transparency

These two terms describe the quality of the sky for observing. Transparency is how clear the air is (free of haze, dust, or clouds). High transparency is needed for faint objects. Seeing is how stable the atmosphere is. Good seeing means the air is calm, and stars appear as sharp, steady points of light, allowing you to see fine detail on planets or in galaxies.

Star-Hopping

The fundamental technique for finding objects in the night sky without a computerized telescope. It involves using bright, easy-to-find stars as signposts to create a path, or "hop," to your desired faint object. Our "How to Find M88" guide is an example of a star-hop.