The Celestial Sphere

by Matthew Johnson

The planet Mercury is always a challenge to view, but this May will be the best month to see it during the year 2021. Mercury is the closest planet to the sun, so it can only be viewed at dusk just after the sun sets or at dawn just before the sun rises.

On May 3, if you can find an unobstructed horizon, look for Mercury 45 minutes after sunset in the west-northwest. Mercury will be just to the left and below the Pleiades (Seven Sisters). Having Mercury next to the Pleiades assists in locating the tiny elusive planet.

The moon and Venus are the celestial sphere’s two brightest objects. When they are close to each other, it is a sight to behold, and something an astronomer looks forward to. At 8 p.m. just 25 to 30 minutes after dusk on Wednesday, May 12, the moon will pass closer to Venus than it has in more than a year. The moon will be a tiny crescent, and it will be about one moon diameter away and to the left of Venus. This will be an impressive spectacle. Look west-northwest low in the twilight for this pair.

On Thursday, May 13, the moon, having traveled eastward in its orbit for another 24 hours, is now alongside Mercury. Having the moon in proximity to Mercury is another great way to locate the tiny planet. Look to the west-northwest at dusk 30 minutes after sunset. The thin crescent moon will be just to the left of the planet Mercury. 

There will be a total lunar eclipse just before dawn the night of May 26. However, because of our location we will not be able to observe the eclipse reaching “totality” (the moon at its darkest). We on the East Coast will experience just a few minutes of the moon’s dimming as it enters the Earth’s shadow. Unfortunately, this takes place about a half-hour before the sun rises, so we have just a few minutes to view the eclipse before it’s lost in the dawn’s glare. The moon will pass through the center of the Earth’s shadow, and for 18 minutes the moon will darken somewhat, most likely appearing a reddish brown. 

The farther west one is from the East Coast, the longer one can view the eclipse. To view the entire eclipse, you’d have to be located onboard a ship or castaway in the Pacific Ocean between Hawaii and Australia. 

We conclude this month’s observing on May 31 by once again using the moon as a guide, but this time it is with Jupiter and Saturn. Look high up in the south-southeast just one hour before sunrise to find a half moon just under and slightly to the left of Saturn and about seven moon diameters below and to the right of Jupiter. Remember, with a simple pair of binoculars you can see Saturn’s ring and Jupiter’s four largest moons. 

Moon terminology: 

Waxes: The moon waxes or is waxing during the phases from new moon to the full moon—the illuminated area increases in size each day.

Wanes: The moon wanes or is waning during the phases from full moon to new moon—the illuminated area decreases in size each day.

A gibbous moon is any phase when the moon appears more than half-lit but less than full.

Phases of the moon :

May 3: Last quarter (left half of the moon illuminated). Moon is waning. 

May 11: New moon (no visible moon). The best time of the month to observe faint objects such as distant galaxies. 

May 19: First quarter (right half of the moon illuminated).

May 26: Full moon. 

May’s full moon is known by Native American tribes as the Flower Moon, owing to the abundance of flowers in most regions. It was also referred to as the Corn Planting Moon and the Milk Moon.