When the sun sets on Longs Peak
— Editor’s note: This Johnnie St. Vrain answer appeared originally in
2005. Given the recent interest in celestial events, Johnnie calculated that this would be a good “best of” to republish. Considering that April 15 already is considered Tax
Day, Johnnie proposes that Aug. 26 be proclaimed as Longs Peak Day in Longmont.
DEAR JOHNNIE >> I think there should be two days each year that Longmont residents celebrate “Longmont Day,” but I don’t know the two dates. These days are scientifically predictable. Can you find a scientist to identify them for us? They should be the days — one in the spring, one in the fall — that the sun sets directly behind Longs Peak as viewed from the intersection of Main Street and Mountain View Avenue. Even if these don’t end up being celebration days recognized by the city, I’m sure many would find it interesting to note these dates for celestial observation. — Dave Miller
DEAR DAVE >> Many thanks to Keith Gleason, the manager of the Sommers-bausch Observatory, Department of Astrophysical and Planetary Sciences, University of Colorado ( cosmos.colorado.edu/sbo), for the answer to this intriguing question. I asked Gleason to let us in on his calculations.
Gleason says: “We want to ascertain those dates for which the sun can be seen 3 degrees, 25 minutes above the horizon, while simultaneously at an azimuth bearing of 180 degrees, 16 minutes, all from the vantage point of a person standing at the previously determined latitude and longitude of the intersection of Mountain View and Main in Longmont.
“And now to the astronomy part of the problem! Here is where we get to ‘cheat’: Put away the calculator and pen and pencil, and drag up our old reliable, tried-and-true Answer Machine
— the Voyager II planetarium program from Carina Software.
“After keyboarding myself to be standing in the middle of the street in Longmont, I fixed my electronic gaze at the precise calculated direction of Longs Peak, and let time run its course, waiting for the sun to traipse nearby.
“After a few minutes’ worth of taking temporal trips several years into the past and back to the future, one watches the sun (at a fixed time of day) describe a figure-eight pattern in the sky known as the ‘analemma’ — the top and bottom of the ‘8’ being the northernmost and southernmost annual excursions of the sun (summer and winter solstices, respectively); the width of the ‘8’ reflects the ‘equation of time’ where the sun runs fast or slow compared to the time on our watches because of effects of the earth’s axis tilt and its orbital eccentricity.
“Or, on any given day, one can watch the sun appear to slide downward and northward towards the horizon, to finally set in the west (combining the effects of earth’s tilted axis and its daily rotation).
“And then one can isolate the two moments in time each year that are the ones being sought: In the spring, the sun center will be most closely aligned with the tip of Longs Peak at approximately 7:16 p.m. MDT April 15, and in late summer, the sun will again line up with the peak at 7:18 p.m. Aug. 26.
These times and dates may change by as much as one day (forward or backward) or one minute (earlier or later) because of the effects of a leap year; the sun doesn’t exactly repeat itself from one year to the next. But the changing date doesn’t really matter too much, because the half-degree-diameter sun appears — at the distance of Longs Peak — to span a diameter of more than 1,200 feet. Thus, if the sun misses Longs Peak slightly too far to the south on April 15 of a particular year, for instance, one can always walk northward a block or two to make it hit ‘dead on’ on that date. Without a doubt, I’m positive that someone walking up and down Main Street between Ninth Avenue and 17th Avenue will always be able to see the sun set behind Longs Peak on April 15 and Aug. 26 — provided, of course, that the sky is clear and there actually is a good view of the mountain!”
Wow. Thanks, Keith, for the work — not to mention the education — that went into that answer.
— Send questions to johnnie@times-call.com.