Spring is a great time to observe a fascinating phenomenon called earthshine, also known as “the moon’s ashen glow.”
The moon goes through a cycle of phases as it orbits the Earth. A few days before or after the new moon, a slender crescent is visible in the sky. But if you look closely, you will be able to pick out the unlit portion of the lunar disc. This is called earthshine, which is more obvious in photographs where the faint outline of the entire moon is clearly visible.
The illuminated part that we see is light that the moon has reflected from the sun. The same applies to Earth: Approximately 37% of the sunlight that reaches our planet is reflected back into space by the clouds and the oceans. Some of that reflected sunlight then lights up the moon. This light reflected from Earth is much fainter than direct sunlight, however, so when the sunlit portion of the lunar disc is large (first quarter, last quarter, gibbous or full moon), earthshine simply gets swallowed up in the sun’s glare. You can only catch earthshine when the moon is a thin crescent.
The period around the spring equinox, from February to May, is the best time to observe a thin crescent moon in the twilight sky. At this time of the year, at our latitudes, the ecliptic(the imaginary line that marks the path of Earth’s orbit, along which the sun, moon and planets appear to travel through the sky) makes a very steep angle with the horizon after sundown. This favourable viewing geometry allows the very young moon (18-48 hours after the new moon) to rapidly pull away from the sun and the horizon and achieve good height at twilight.
There will be a wonderful opportunity to observe earthshine on May 6 and 7, about 45 minutes after sunset. On the 6th, the crescent moon hangs just above the bright star Aldebaran in the constellation Taurus, low in the western horizon. The following evening, the moon will rise higher in the sky and park itself to the lower left of Mars, recognizable by its red hue. While earthshine is readily visible to the naked eye, it is even more spectacular when viewed through binoculars or a small telescope.
Check out debris from Halley’s comet
Also visible in early May will be the Eta Aquarid meteor shower, which peaks over several days around May 6. Viewing conditions should be excellent since the new moon falls on the 4th. However, its radiant point in the constellation Aquarius will appear above the horizon only after 3 a.m., so the window of observation is limited to the wee hours before dawn.
Although not as famous as the Perseids in August, the Eta Aquarids typically produce fast-moving, bright meteors. These streaks of light are created by debris, such as dust and tiny pebbles, smashing into the Earth’s atmosphere at nearly 67 kilometres per second. In this case it’s debris shed by Halley’s Comet into its orbital stream, as it swings by the sun once every 76 years.While Halley’s comet isn’t expected to return until July 2061, you can still watch bits of the famed celestial body zip across our sky as Eta Aquarid meteors.
The planets in May
Mars is still visible in the early evening. Look for the planet low on the west-northwestern horizon about 45 minutes after sunset, where it remains observable until it sets around 11 p.m.
This is the month Jupiter finally joins Mars in the evening sky. Shining brightlyin the constellation Ophiuchus, the giant planet rises in the southeast around 11:30 p.m. in early May, then increasingly earlier with each passing day; by the end of May, it rises around 9:30 p.m., barely an hour after sunset. Saturn currently rises two hours after Jupiter and, as such, is only visible in the second half of the night.
At around 2 a.m. on May 20, the waning gibbous moon will be to the right of Jupiter, above the southern horizon, and then to the left of Jupiter the following day. At the same time on May 22, the gibbous moon will hang to the right of Saturn, and then to the lower left of the ringed planet on the 23rd.