May 30, 2020

Determine your Longitude: the Lunar Eclipse Method Part II

Hopefully, you had clear skies at your locale. My luck was pretty good… mostly clear skies through-out! My initial impressions were that of a very bright eclipse; the southern rim of the moon seemed especially bright. The color ranged from a dark blood red on the northern edge to an overall brownish glow. This seemed particularly prominent through binocs. And it was extremely cold! Temps ranged around zero Fahrenheit. The night was even punctuated by a fast pass of spy satellite USA 193, on what turned out to be its final orbit. So much for a scoop by Astroguys…

Now for last week’s experiment;

Here is a brief synopsis of the timing results;

Timing after sunset-Naked eye estimate-Nocturlabe Reading-Actual Time

1st contact(umbra)                  8:49PM   8:42PM  8:26PM  8:43PM

2nd contact(totality begins) 10:05PM   9:59PM  9:26PM 10:01PM

3rd contact(totality ends)     11:01PM 10:55PM       N/A   10:51PM

Converting the timings after sunset, we come up with:

1st contact= 73.5 degrees west

2nd contact= 74 degrees west

3rd contact= 72.5 degrees west

As you can see, the sunset and estimated times were pretty accurate, the nocturlabe less so.  What happened? After all, this is science… surely some lessons can be learned… The magic longitude number is about 68 degrees west for Saint Froid Lake in Northern Maine where our humble abode is located. 75 puts us in Ontario some where. Not great for navigating the high seas. And I won’t even publish the nocturlabe results, as they were even further off the mark.

I suspect three things may have been at work with the sunset timings; first, we do not have a perfectly flat horizon to measure sunset by. I had to guess-timate the exact local time of sunset behind a hill to the northwest. Second, elevation above sea level (ours is about 5oofeet) may have been a factor. Lastly, I’d place a small bet that the margin of error can be expected to be greater using this method in more northerly latitudes.

As for the Nocturlabe, I can say from use that its extremely difficult to get an accurate reading in the cold, with gloves on, in the dark! Hats off to those old navigators that did it from the pitching deck of a ship… still, the readings were consistently off! I’ll leave this open to anyone who is interested as to why…

A link to a short video I composed during the eclipse, complete with cool Bjork music, can be seen at; Please note that in order to protect my camera from the arctic climes,  I could only shoot a few minutes at a time!

Other interesting results- I estimated the Danjon number of this eclipse to be about 1.5. That is, a deep red to brown, with a bright limb. Using the patented “reverse binoc” method, I estimated the brightness to be stellar magnitude +2.15. I used the nearby stars Regulus, Denebola, Algieba, and the planet Saturn for comparison. I knew I bought those $1,000+ Canon image stabilized binocs for a reason!

The curious tale of Columbus and the eclipse of 1504. Speaking of Eclipses, It turns out that ol’ Chris wasn’t above twisting science to his own ends, or at least to get his own way. Legend has it that in Jamaica in 1504, he did just that!  Using his knowledge of an impeding eclipse, Columbus successfully frightened the natives into submission. A recent article on this can be fond at;

Perhaps the above experiment shows why Columbus believed he had landed so far off course… I hope many of you braved to cold February night to witness this eerie spectacle. There won’t be another Total Lunar Eclipse visible from North America until 2010!


  1. [...] chilly lunar eclipse- On the home front, the February total lunar eclipse was probably the coolest event we personally witnessed this year. The temps were bitter, but the [...]

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