You can have an enjoyable observing session any time the Moon is in the sky, but for the absolute best telescopic images, it pays to know which lunar phase offers the greatest potential for high-resolution views.
Because the Moon is so bright, it’s well seen even under the most light-polluted city skies. Indeed, the main factor that limits what you can see in your telescope is something astronomers call seeing. This term refers to the steadiness of the air above us. Ever-present turbulence in the Earth’s atmosphere causes distortions that make the stars appear to twinkle. In telescopes, the effect can produce a waviness in the image or a fuzzy view. But when the turbulence is minimal and the seeing steady, your telescope can deliver its sharpest views.
The atmospheric turbulence that affects seeing occurs at various levels, but most of it is either close to the ground (local seeing) or hundreds or thousands of metres overhead (high-altitude seeing). Local seeing conditions are the product of many variables, some of which are difficult to predict. However, you can stack the deck in your favour by taking a few simple steps.
First, make sure you’ve allowed plenty of time for your scope to cool to the ambient air temperature. A small refractor takes just a few minutes, while a small to medium reflector requires 30 to 60 minutes, and a large reflector (more than 6-inch aperture) takes even longer. Second, avoid aiming your scope over known sources of heat, such as a house or a paved parking lot. Third, know when to look.
The optimal time to view the Moon — or any celestial object, for that matter — is when it’s highest in the sky. On most nights, that’s when it crosses the meridian, the imaginary line that passes directly overhead, joining due south to due north. Seeing conditions generally improve as the amount of air you look through decreases. And because the thickest cross section of air — and, consequently, the most turbulence — lies nearest the horizon, you’ll get a sharper view of the Moon when it’s high in the sky.
The quality of your view also depends on where the Moon is situated along the ecliptic, the path the Sun, Moon and planets trace across the sky. When the Moon is parked in Sagittarius (the ecliptic’s southernmost extreme), the lunar disc crosses the meridian less than 20 degrees above the southern horizon for most Canadians. That’s low enough that you’ll be looking through a lot of air and unlikely to enjoy steady seeing.
For Canadian astronomers (or anyone observing from north of the Tropic of Cancer), the Moon reaches its absolute highest point in the sky on nights when it lies in the northernmost reaches of the ecliptic. Thus at any specific time of year, there’s only one phase of the Moon that’s optimally placed.