National Astronomical Observatory of Japan

The Moon Approaches Close to Venus, Mercury, and Mars (September, 2017)

September17-19, 2017, 30 minutes before sunrise, the sky of Tokyo
Medium resolution (2000 x 1265) High resolution (5500 x 3480)

Try Observing the Extremely Thin Moon before the New Moon!

In mid-September, Venus stands out brightly in the eastern sky just before sunrise when the sky is bright. And to the lower left of Venus, Mercury, which reaches its greatest westward elongation on the 12th, is relatively easy to find. On September 18 and 19, the Moon, waning towards the new moon on the 20th, approaches close to Venus and Mercury in turn.


First, the 27 day old Moon approaches close to Venus on the 18th. About an hour before sunrise, the altitude of Venus has already exceeded 10 degrees. The thin Moon is difficult to see in the sky which has started to glow. Therefore while it’s still early, let's look for the Moon using Venus as a guide from a location where you can see the eastern horizon unobstructed. When the sky is still dark, it will be easy to find even a thin moon. You can also observe the phenomenon where the dark side of the Moon is illuminated by light reflected from the Earth. This is a phenomenon called "earthshine".

After that on the 19th, the Moon, which becomes much thinner and lower in altitude, approaches close to Mercury and Mars. The 28 day old Moon is said to be the phase limit to find the Moon. Even half an hour before sunrise, the altitudes of Mars and Mercury are less than 10 degrees. But first let's find Mercury and check it with binoculars. Then, if the conditions such as the weather are good, in the field of view you might find the thin Moon, as if it had been wedged snugly between Mars and Mercury.

You can look up the times when the Sun rises at various locations in the Koyomi Station of the Ephemeris Computation Office, NAOJ.

Let's try observing the difficult to find Moon just before the new moon. This is a good time to observe because the planets can be used as guides.

Reference: Ephemeris Computation Office

With the β€œSky Viewer” you can easily explore the appearance of a typical urban night sky (planets and constellations are visible). The Celestial Phenomena section of the glossary explains the planetary phenomena terms: greatest elongation, opposition, conjunction, stationary, etc.