Encircling the Vernal Equinox


In early January, I wrote about the cosmic grid of right ascension and declination. Declination is cosmic latitude measured north and south from the Celestial Equator to plus 90 degrees and minus 90 degrees. Right ascension, cosmic longitude, is measured from the Vernal Equinox to the east in hours where one hour equals 15 degrees of arc on the sky (because the Earth rotates 15 degrees each hour).

As shown in the figure, Venus, Mars and the moon nearly encircle the Vernal Equinox tonight, giving us a perfect opportunity to see this important position on the sky.

In the coming days, the moon will quickly move away from the planets as its orbit carries it about 12 degrees to the east every day. As it does, more of the side of it facing the sun will be visible to us, making the slender crescent of tonight thicken as it moves. To teach my students about the phases of the moon, I give them all oranges and suspend a bright light bulb in the middle of the darkened classroom that they all face. While holding the orange in one’s left hand, each student starts with it between him or her and the bulb. This is the “new moon” phase, where only the dark side faces Earth. This will be the situation on Aug. 21, when the Great American Eclipse will occur and the moon will only be visible by the light of the sun it blocks. Most recently, the new moon occurred at 7:07 p.m. on Saturday, Jan. 27.

To view the waxing phases, holding arms outstretched, each student slowly swings his or her orange to the left so the lit side of the orange comes into view as a slim crescent that grows as one continues to move it. When the orange gets to the student’s side, 90 degres from where it started, half the lit side is visible, giving the appearance of the first quarter moon. It’s the first quarter because half of the half of the moon we can see is lit, and it’s one quarter of the way around the Earth. This phase will occur on Friday night, Feb. 3 at 11:19 p.m.

Beyond the 90-degree point, more than half the orange is lit, imitating the waxing gibbous phases. The name “gibbous” is from the Latin word for hunchback. Once the moon is opposite the lightbulb and the students have all turned their backs to the light, the entire face of the orange is lit, as is the full moon. This phase will occur next the following Friday, Feb. 10, at 7:33 p.m. If the students hold the oranges in the shadows of their heads, they experience a lunar eclipse. There will be an eclipse on Feb. 10. The eclipse will begin just after moonrise, but you won’t notice because it’s a penumbral eclipse.

Note on the diagram that the moon is close to the Ecliptic, the Earth-sun plane, tonight, but then moves south of it as it waxes toward full. Thus the moon will only pass through the southern part of Earth’s shadow, where only a portion of the face of the sun is blocked, making it only 2 percent dimmer than if it weren’t eclipsed. Next year, on Jan. 31, 2018, the western U.S. will be able to view a total lunar eclipse.

We’ll be able to see the beginning of it, but the moon will set before totality. The next total lunar eclipse we’ll be able to see from the Adirondacks will occur on Jan. 21, 2019. It will start at 10:44 p.m. and be total from 11:41 p.m. to 12:43 a.m.

Continuing on the journey of our orange as it continues past full, it goes into the waning gibbous phases until it gets to the third-quarter phase. This occurs when the moon has only one-quarter of its orbit left before it aligns with the sun again. The next third-quarter moon will occur at 2:34 p.m. on Saturday, Feb. 18. After that, the orange, and our real moon, move into the waning crescent phases. These are the beautiful crescent moons many of us are treated to on dark winter mornings as we leave home to head into work.

Few of my students have ever really looked at the moon. Yes, they’ll glance at it but disregard it as one of our constant companions. Hence one of the observing assignments I give them is to look for the moon at the same time every night for two weeks while it goes through the waxing phases. Though we rarely have clear skies for two weeks, a little deliberate attention to its phase and position can connect us to this nearby world in a new way. I hope you’ll pause and really look at the moon over the next few nights to watch how it changes as it orbits our home world.

The astronomers of the Adirondack Public observatory will be happy to share views of the moon and other wonders of the night sky from our RORO (Roll-Off-Roof Observatory) above Little Wolf Pond in Tupper Lake. We’re open to the public on the first and third Fridays of each month, weather permitting. For updates and notices, check out adirondackpublicobservatory.org and our Facebook page. On our public observing days, you can also call the RORO at 518-359-6317 to talk with one of our astronomers. Observing starts about one half-hour past sunset.