February 23, 2020

Viewing a Low Altitude Occultation

This past Thursday, I got an e-mail from Sky & Telescopes’ automated alert system;†Monday, the 18th of June, there would be an occultation of Venus by the Moon visible from extreme northern New England and the Canadian Maritimes.† This occultation would also span the Atlantic, Europe, and into Asia, but would be especially difficult to spot from the continental US (what we in the miltary refer to as ConUS) due to its extremely low elevation in the†day time sky.

A call was sent out for any one†able to observe this unique event to send in reports.†We currently live along the shores of Saint Froid Lake in†extreme northern Maine; I soon realized that we may have a shot at seeing this!†† Problem #1- Would the Moon and Venus be visible at all? The event†was scheduled for†09:05AM EDT, well into daylight.† The Moon was only 3 days old, and tough to spot visually being so close to the sun. Also it would transpire only 6 degrees above the†north eastern horizon; trees and low lying clouds could easily thwart efforts. Atomospheric extinction can†also cause a faint cresent†Moon to vanish entirely.

Problem #2- Did I have the proper equipment to view the event?†Certainly, a “GOTO”†drive telescope would be preferable; I could just align it the night before and†leave it set up for the next morning.† However, both of my telescopes with this capability are currently in storage.† What I did have is a pair of Canon 15×45 Image Stabilized (IS) binoculars.† With a generous field of view, these rival a small telescope.† They cannot be tripod mounted, but the image stabilization allows the view to “float” before the observer, allowing much finer detail to be seen.† The military now uses these in Afganistan; A-10 pilots carry them for ground spotting enemy positions.

I only had a few days to prepare; I decided to break down the†task into a series of observations.† The first challenge; can the†three day old cresent moon be spotted in the daylight?† I decided to try to see it Saturday, not quite 48 hours prior to the event.† The moon was higher in the sky, thus an easier day time target and not prone to atmospheric extinction; however the cresent would be much thinner.

Patiently, I scanned the imaginary line of the ecliptic, the path that most solar system objects, including the moon, follow.† I did this from the shadow of our house, thus blocking the sun. I’ve used this simple method with success to spot Venus, Jupiter, and even Mars†in the daytime. Knowing the position of†your quarry in relation to forground objects, such as a post or flag pole, also helps.

After about five minutes of slow scanning, I picked up the faint ghost like image of the Moon. This confirmed that at least it would be posibble to spot†a very young moon in broad daylight.

The next observation occured several hours later after sun down.† The focus; would the position of the beginning of the Venus occultation be†unobstructed by my local horizon?† Our property sits on the throughfare between Eagle and Saint Froid Lake; we are on the western shore, so our horizon over the water from the northeast to the south is fairly low.

To answer this question, some before hand research was in order.† I wanted to use a fairly bright star that would pass near the same position as a marker.† For this, I used a freeware planetarium called HNSky.† Any planetarium program would suffice, but I like HNSkys’ simple to use interface. Using this program, I simulated the sky from my location about an hour after sunset.† I had already used it earlier to simulate the position of the occultation. I discovered the 2nd magnitude star, Enif in Pegasus would only be a few degrees from the occultation position.† Looking northeastward with my binocs, I discovered Enif hovering just above the trees. I sketched the horizon†noting the brighter stars and mentally locked the approximate location in my brain.

My next step was to try and sight the Moon and Venus the morning before the event.† I scanned the horizon for about 10 minutes, but a scud of low altitude clouds thwarted my attempts.

The morning of the big day dawned†blue and sunny, as the Twin Dog Observatories’ online Clear Sky Clock said it would.† I knew that we would need an absolutely crystal†clear sky to have a shot at success; even a distant layer of clouds would ruin any chance of seeing†ingress.†† I set up the following equipment;† a JVC digital video recorder, a digtal voice recorder, and†an AM short wave radio.† Initially, I had plans to use the traditional method of having WWV calling out a precise time via short wave while the video and audio recorder were running;† however, I could not recieve a strong WWV signal.† My alternate method consisted with syncronizing my watch†to an online time site. This will gain an estimated accuracy†of about a second; If I could catch the event on video, a higher†accuracy could be attained.

Precisely timed†occultations can be used to†deliver useful scientific information; I series of observers along a graze line can produce a map†of the lunar limb in profile.† close†double stars have also been discovered during occultations, their light “stepping out” in close succession.† The main challenge for this particular occultation would be to test my own observing skills plus strain visual observations to their limit, not bad for a Monday morning!

I started scanning the horizon at 08:30 AM EDT; nothing but a white-blue horizon.† I scanned again at 08:45; after about two minutes I picked up a faint fingernail shape emerging from behind a pine tree; on its dark edge, a small diamond sparkle; Venus!††Neither was visible to the†naked eye; ironically, Venus was much brighter than the moon through binocs. I almost had to use averted vision to even spot the low contrast moon; once I did, Venus popped right out.

At 08:55†I started the video and audio recorder running; the image didn’t appear in the view finder but I centered it as best I could. At about 09:02 I started full time visual scanning.† Shortly before 09:05 a wonderous thing happened; Venus began a perceptable fading.† After about ten seconds, it had completely vanished.

Later that morning, re-imergance was much easier to spot.† The moon was higher now,†and both it and Venus were†visible to the naked eye.† I snapped a quick succession of photos,†and†pointed out the daylight pairing to anyone who would look up.

And the lessons learned?† Low altitude†occultations are observeable, provided that the objects concerned are very bright and your sky and horizon is clear. Across Europe and Asia they no doubt had a beautiful view of a twilight pairing; but I’ll bet they didn’t recieve the benefit of achievement we did!

Note: a short video†of†Venus and the Moon after the occultation is visible at:


Final note; after reviewing the video, technical stats for the occultation break down as follows:† local time at the begining of the occultation: 09:05:04 AM EDT (13:05:04 AM UT)† Time of fade out: 10.97 seconds.

A short synopsis of the occultation with pictures can be viewed at:



  1. Albert G says:

    How I add this article to Digg?

  2. MOBY says:

    I can’t add your post to Digg. How I do this?

  3. Daniel says:

    I couldn’t understand some parts of this article Viewing a Low Altitude Occultation, but I guess I just need to check some more resources regarding this, because it sounds interesting.

  4. Bernard Hill says:

    Hello webmaster…Man i love reading your blog, interesting posts ! it was a great Thursday

  5. Intriguing write up on Viewing a Low Altitude Occultation. I love your interesting posts.


  1. [...] amazed at how simple this feat is-a great thing to amaze your friends with. Iíve watched the Moon occult Venus in the daytime from our camp on Saint Froid Lake on northern Maine in 2007, and managed to [...]

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