International Observe the Moon Night Is Tonight

Tonight, Saturday, September 19, the world marks International Observe the Moon Night, an event held every year since 2010. The initiative is coordinated by NASA, with support from the Lunar Reconnaissance Orbiter mission and the Planetary Science division at NASA’s Goddard Space Flight Center. No telescope or dark sky is required: the Moon is visible even from a lit up city, and there is no official viewing hour. Observers can look anytime from sunset until late into the night, on their own schedule.

Why this particular Moon phase

Organizers pick the date each fall in the Northern Hemisphere close to the first quarter phase, because that is when shadows along the boundary between light and dark on the Moon, known as the terminator, stretch longest, making craters and mountain ranges stand out most clearly. This year’s first quarter fell on Friday, September 18, at 4:44 PM EDT (10:44 PM in Belgrade). The Moon also passed through apogee that same day, the farthest point in its orbit around Earth, at 11:00 PM EDT (5:00 AM in Belgrade on September 19), at a distance of 404,221 kilometers from Earth.

Organizers also explain why this phase, rather than the full Moon, works best for observing. When the Moon is full, sunlight falls almost straight onto the side facing Earth, so mountains and craters cast only short shadows and the surface looks comparatively flat. At first quarter, the phase on display tonight, light hits at an angle, so shadows along the terminator make the terrain stand out far more, and even sunlit mountain peaks poking out of the darkness beyond the day night line become visible.

How much of the Moon is lit tonight

Sources do not fully agree on the exact illumination figure, so both are reported here. StarWalk puts tonight’s Moon at about 60 percent lit. Astronomy.com, which tracks the Moon’s phase daily in its weekly column, lists a slightly higher figure for the same evening, measured at 10:00 PM local U.S. time: 62 percent. EarthSky, a third source that follows this event, does not give a specific illumination percentage for September 19, describing the Moon only as waxing gibbous.

What is visible to the naked eye and through binoculars

Tonight the Moon sits in the constellation Sagittarius, with the star Nunki, magnitude 2.1, passing just 0.36 degrees away, closer than the Moon’s own apparent diameter. With the naked eye, several dark plains, known as maria, stand out on the surface:

• Mare Tranquillitatis
• Mare Serenitatis
• Mare Fecunditatis
• Mare Crisium
• Mare Imbrium

Through 10x binoculars, the craters Tycho, Plato and Alphonsus become visible too, along with the lunar Apennine mountain range. Organizers recommend starting with the naked eye, then moving to binoculars or a small telescope if one is available, since each level of magnification reveals a different layer of detail, and too much magnification often just blurs and darkens the image instead of sharpening it.

The event is also meant as a chance to bring people together around watching the sky. Anyone interested can organize their own small gathering, from a get together with friends and family to a virtual program, and share photos and impressions afterward.

What else is happening in the sky this week

A few other events fall in the same week. On September 19, around midnight, Mars passes 6 degrees south of the star Pollux, in the constellation Gemini, bringing the two objects close together in the sky. The autumn equinox arrives on September 22 at 8:05 PM EDT (2:05 AM in Belgrade on September 23), marking the official start of astronomical fall in the Northern Hemisphere.

No organized public event is planned in Serbia for tonight’s occasion, but watching from a balcony, terrace or yard is still worthwhile, ideally with a clear view toward the southeast and south, since observers at northern latitudes will see the Moon tracking relatively low across the southern part of the sky tonight. This is the first time this outlet has covered the event.