M79 Globular Cluster in Lepus: Visual Comparison With M68

How the two clusters look

M79 (NGC 1904)

M79 is a small, compact ball of stars. Its apparent diameter is 7.2 arcminutes, about one-eighth of a degree, so it fits easily in a low-power eyepiece field. With a 114-150 mm reflector, the expected view is a round, softly glowing ball. The outer edge is fairly even, and the glow brightens toward the middle.

At magnitude 8.2, M79 is well below the naked-eye limit of about magnitude 5 in a suburban sky. You will not see it without an optical aid. In a small reflector, the cluster looks like a patch of light with a faint granular texture at the edges. Individual stars appear only at higher magnification and under good transparency.

M68 (NGC 4590)

M68 is also a globular cluster, but its look is different. Observers usually describe it as a loose, diffuse glow in a small instrument, with its outer parts faint and its core less dense than the core of a tightly packed cluster. Compare the two clusters on the same night if you can. Note how the brightness rises toward the center of each one, and where the edge seems to fade into the background sky.

Because M68 sits in Hydra, far from Lepus, the two clusters are not seen together on a single night from most locations. Plan separate sessions, or compare them on the same night only if you can locate both in a single session with a finder chart.

Difficulty in a suburban sky

M79 is not a difficult object for its type, but it is faint enough that suburban light pollution matters. Street lights and sky glow lower the contrast between the cluster and the background. A dim, diffuse object like this loses contrast faster than a bright star does.

Start with the instrument's finder. Locate the cluster with low power, then increase magnification to see its shape clearly. Use averted vision: look slightly to one side of the cluster so the light falls on the more sensitive parts of your retina. Dark adaptation takes about 20 to 30 minutes, so avoid looking at phone screens or bright lights during that time.

Expect M79 to look most like a ball at low to moderate power. Higher powers spread the light and make it dimmer, though they can show the outer stars more clearly if the sky is steady. Typical magnifications for a 114-150 mm reflector range from about 60x to 120x for the cluster, depending on conditions.

M68 is generally regarded as the more demanding of the two in a suburban sky, because its light is more spread out. Under light-polluted skies, you may need a dark site to see its outer edge.

Timing and altitude by latitude

The best months for M79 are January and February. In those months, the cluster is well placed in the evening sky. Its maximum altitude depends strongly on your latitude, and this changes how clearly you can see it.

• At 35° N: M79 reaches a maximum altitude of 30°. At this height, you look through a considerable amount of atmosphere, so the image may twinkle and blur. Observe it as soon as it is well above the rooftops and trees.
• At 50° N: The maximum altitude drops to 15°. At this height, the cluster sits low, and haze, horizon glow, and local obstructions can hide it. Views are often poor, and you may only catch it on the clearest nights.
• At 35° S: The maximum altitude is 80°. The cluster passes nearly overhead, where the atmosphere is thinnest. Views are usually steadier, and it is easier to find the cluster and hold it in the eyepiece.

For observers in the Northern Hemisphere, M68 is a spring target and is better placed in the evening sky of that season. The timing difference matters for planning. A winter session suits M79, and a spring session suits M68.

Technique: how to find and observe each cluster

Finding M79

Use the stars of Lepus as your starting point. Arneb, the brightest star of Lepus, lies to the northeast of M79, at a distance of several degrees. Nihal, Beta Leporis, lies to the northeast as well, about 4 degrees from the cluster. From Nihal, move a few degrees south and slightly west to find M79. Use a finder with a wide field first. Once the cluster is in the field, center it and switch to a higher-power eyepiece.

Because the cluster is faint, use a star chart to confirm that you are in the right area. Sketch the surrounding stars with the finder before you move on. A simple chart helps you return to the cluster on later nights.

Finding M68

Start from a bright star in Hydra and sweep the area with a low-power eyepiece. Star hop from known stars to the cluster, marking each step on a chart. Because M68 is fainter and more diffuse than many bright targets, a slow, careful search works better than a fast one.

What the technique teaches

Observing M79 teaches you how a compact cluster responds to changes in magnification and dark adaptation. Observing M68 teaches you how to judge a diffuse, loosely spread glow against a faint background. Practice both, and you will learn how a cluster's shape, density, and brightness affect what you see.

What each cluster teaches

M79 is a good target for learning how to hold a faint object steady and how to use averted vision. Its compact shape makes it easy to recognize once you have found it. From the Southern Hemisphere, its high altitude makes it a convenient target for practicing these skills on a steady sky.

M68 teaches you to recognize a looser cluster and to judge its edge. Its diffuse light challenges your sense of contrast, and comparing its look with M79 helps you learn how the density of a globular cluster changes its appearance.

Observing both clusters, in sessions planned around their different seasons and altitudes, gives you a clearer sense of what these objects look like than observing either one alone.