A good military operations plan considers the times of sunrise, sunset, and the nighttime illumination offered by the moon. I see no reason why a historian approaching the study of an operation or action shouldn’t also consult these factors. (Since the data is often easy to come by, I “deduct points” from any review if an author fails to address these factors!)
The following chart was derived from data from the U.S. Naval Observatory specifying the year as 1863 and the place as Leesburg, Virginia:
The day of the month of June is listed on the far left column, and data for that particular day is listed on the row to the right of the day. For those who are not familiar with the abbreviations across the top, here the translation:
BMNT – Beginning Morning Nautical Twilight. The sun is 12 to 6 degrees below the horizon. Weather and other factors aside, the first traces of illumination from the sun are apparent. One can start making out ground objects at BMNT, but stars are still visible.
BMCT – Beginning Morning Civil Twilight. Defined as when the sun reaches 6 degrees below the horizon. Illumination increases to allow one to fully recognize objects on the ground, but only the brightest stars are visible.
Sunrise – Defined as when the sun is visible arising over an unobstructed horizon – not over terrain such as hills.
Sunset – Similarly, when the sun disappears below an unobstructed horizon.
EECT – End of Evening Civil Twilight – The sun passes beyond 6 degrees below the horizon. As with morning, between sunset and EECT, illumination allows one to see objects on the ground, and some stars are visible.
EENT – End of Evening Nautical Twilight – The sun passes beyond 12 degrees below the horizon. At this point, no ground objects are made out without artificial light or moonlight.
Moonrise and moon set are defined similarly to sunrise and set. Note the table above adjusted the times for some moon sets to correlate the span of the moon’s visibility “overnight” for a particular day.
Moon phase – Traditional terms used to define the phase of the moon.
% Illum – Is the percentage of the moon’s surface that is illuminated. From an operational standpoint, this gives an indication, again weather aside, of how much reflected light will aid those operating at night.
Those definitions set, the table offers some indication of the working daylight hours and the impact of night operations around Edwards Ferry in June 1863. Based on the data, the earliest that soldiers might start work was roughly 4:15 a.m. each morning. The terrain east of Edwards Ferry features a few hills, but nothing that would greatly obstruct the horizon. However, to the west are the Catoctin Range, just outside Leesburg. Given that on the horizon, probably the latest soldiers might work without aid of lanterns or candles was 8:00 p.m.
The moon had risen before sunset on each day reviewed. However, the moon offered very little illumination over the nights of June 20-24, and sat well before sunrise. On the vital nights of the crossing, June 25 – 27, the moon sat a few hours after midnight. Of course cloud cover from the rains likely suppressed any illumination offered by the waxing moon. Thus the infantry who marched beyond the bridges along the tow-path and towards Poolesville, Maryland were not offered much natural lighting.
Of note, on the night of June 20-21, when the engineers placed the first pontoon bridge, the moon sat about three hours after sunset and offered only 17% illumination (assuming no cloud cover). Recall from the timeline that the order to place the bridge was issued at 5:20 pm on June 20. The bridge was completed at 9:45 am the next day. And this occurred while the Potomac was rising!
Perhaps to elaborate on that point with a visual, from my files of “photos that didn’t work out too well” is one taken on June 16, 2007 at 8:47 pm of White’s Ferry:
For the record, the Naval Observatory’s data for that day provides a sunset of 8:39 pm and a EECT of 9:11 pm. The moon was a waxing crescent with only 8% illumination, and sat at 11:15 pm that night. The camera, my simple point and shoot, had no extra filters and the flash of course did little good. The ferry boat is visible to the right and the ramp in the distant center. About the only thing standing out are the headlights of vehicles and a few on the boat.
Can you imagine walking across a swaying bridge in that lighting condition? How about building one?