Occultation of a Sapphire

Out of the 12 zodiac constellations, only six stars shine at first magnitude. Of these, just four lie close enough to the ecliptic for the Moon to occasionally pass in front of them during its monthly orbit. Up until now I had been able to catch occultations of Antares and Aldebaran, but not Spica and Regulus. Lunar occultations of this stellar quartet is not routine. First, the Moon's distance from the ecliptic has to match that of the star when the Moon is swinging by it. Then you have to be in the right location on Earth. Due to the Moon's closeness to the Earth, parallax is in play so that an occultation in the US can easily be a near-miss in Canada. Add to this the need to have clear skies and it's easy to see why many amateur observers have not caught occultations of all the candidate bright zodiac stars.

This month in the pre-dawn skies on Wednesday, November 27^th, Spica was slated to undergo an occultation from my location. Some aspects of the event were very favorable. The Moon would be a fat crescent, making it easier to see Spica adjacent to it. The altitude would be at least 20° so that it would be above the tree line. On the downside, the reemergence from behind the Moon's dark limb would happen close to sunrise, making it impossible to see naked-eye but certainly doable in binoculars. 

The weather prediction on the evening of the 26th was very guarded. Depending on the source, predictions were for anywhere from 20% clouds to 70% clouds with a storm system heading our way for the holiday (Thanksgiving). Given the uncertain weather, I opted not to set up my telescope. Instead, I relied on my binoculars and camera with a telephoto lens for a simpler approach.

I got up around 4:30 a.m. and peeked out the window to see a fair amount of clouds, but a few "sucker holes" thrown in just enough to entice me to getting dressed and going outside. A little before five I was outside and could see the Moon rising through the denuded tree across the street from my driveway. Spica was visible as well, and its blue color evident nicely in the binoculars. But the clouds, not so thick as to obscure the Moon yet heavy enough to degrade the view, were passing by frequently. It'd be a matter of luck as to whether I'd catch anything.

Crescent Moon, Spica clearing tree line

In the back yard I positioned myself where the Moon would be above obstacles and worked on the camera's focus at 300mm. I snapped a few shots and examined them to verify that I had a reasonable focus. As the minutes ticked down to the anticipated disappearance at 5:34 a.m., I alternated admiring the view with just my eyes and then with the binoculars, all the while grabbing a few shots to document the approach. Part of what makes an event like this so suited for visual observation is the ability of our eyes to handle a wide range of brightness. I could appreciate the bright star hanging below the even brighter crescent which then gave way to the dark side of the Moon illuminated by earthshine. It is very hard to recreate that visual experience in a photo.

Finally with one minute to go I switched the camera to record video and inspected the view in my 7x50 binoculars. The task of witnessing the coup de grâce was challenging. My hands were not steady enough to get anything but a fleeting, occasional sighting of the star adjacent to the bright lunar limb. Within a few moments it was gone from view, and I halted the recording and went inside to warm up.

With about 70 minutes to go until reemergence of Spica I had to decide how to document that event. With the clouds continuing to hang around I didn't feel it was worth the effort to set up a telescope to capture it. The camera with its lens would hopefully be able to pick out Virgo's lucida when it popped into view. 

I took my position a little before quarter to seven o'clock and focused the camera on the Moon. As fate would have it there were actually very few clouds now, making me wish I had set up the Vixen to observe the reemergence. With about a minute to go I turned the video on and began targeting the Moon in my binoculars, trying to steady them by resting my elbow on the fence. Suddenly there it was, a shining dot backlit by a strong twilight sky. The show was complete and I brought the camera inside.

Spica's Reemergence

Reviewing the captures revealed the shots leading up to the disappearance went well for the most part, but the last few seconds on video are quite hard to discern the moment the star winks out given the brightness of the lunar limb. The reemergence video is better in documenting how Spica suddenly appears in the pale blue sky. Undoubtably there's much opportunity to massage the digital data on upcoming cloudy evenings into something that more closely represents what was seen by the human eye.

While conditions were not perfect, they were good enough that I’ve now witnessed three out of the four first-magnitude stars in the zodiac disappear behind the Moon ‐ only Regulus remains! This occultation, with Spica glowing like a sapphire alongside the Moon’s western limb, is now one of the highlights of my astronomical year and a reminder of how the cosmos never ceases to delight me.