• Luke

Major Lunar Standstill

Basically Major Lunar Standstill is an 18 year cycle of the moon where the moon moves to the northernmost position it will be in the sky and “stands still” (more or less) for 3 years. This is a big deal at Chimney Rock national monument because it’s when the moon lines up perfectly to rise between the rocks more on that on my blog post here https://www.astrotours.org/post/the-start-of-major-lunar-standstill-season-at-chimney-rock-national-monument

To understand what is happening and how we calculate Major Lunar Standstill we first need to understand “Ephemeris”. Ephemeris is basically the trajectory of celestial objects, it’s kind of like GPS coordinates but also with a time component, so GPS coordinates for a moving object. A most basic example I can think of is let's say there’s a boat floating at sea it so happens that it was dropped in the water right at the coordinates of 0°N, 0°W and is traveling 1° north and 1° west every day if you know it started at 0°N, 0°W on Monday you could predict it will be at 1°N, 1°W on Tuesday, then 2°N, 2°W on Wednesday, 3°N, 3°W on Thursday, and so on. Alternatively if you wanted to see that boat from the shore of Africa at 5°N, 5°W you would know you will have to wait until the 5th day, so on Saturday.

Now ephemeris for celestial objects are not nearly that easy. While it might look to us on earth like the moon is sailing across the sky like a boat in a sea of stars, the Moon is actually orbiting the Earth which is rotating, and in turn orbiting around the sun. All of this as well as your location on Earth has to be factored in when trying to find the moon’s position in the sky.

There are two coordinate systems astronomers use to calculate ephemeris; “Alt, Az” and “RA, Dec”. Alt, Az is the most straightforward as it’s centered on you as you look at the sky. Alt is short for ‘altitude’ but this isn’t altitude like elevation above sea level it is simply degree inclination from the horizon (0°= right on true horizon {like horizon neglecting topography so the horizon you would see in the middle of the ocean or orthogonal to a plumb bob}, 90°=zenith (or directly over head). az is short for azimuth and just is compass angle (0°/360°=north, 90°=east, 180°=south, 270°= west).

The compass angle of the gap of the chimney rocks viewed from the main Kiva is ~53°. Therefore to see the moon between the chimney rocks it needs to be around Az = ~53°. Meaning when the moon rises (Alt=0°) the Moon needs to be ~53° degrees from north, so roughly north west.

Now the tricky part, RA/dec. RA, Dec are like the gps coordinates of all of the sky centered on the earth itself. RA=Right Ascension it is like longitude only instead of 0° being Greenwich it’s where the ecliptic meets the celestial equator {where the plane of our solar system intersects will be the equator}. RA is made even more esoteric in astronomy by being given in hours instead of degrees. This takes some getting use to but makes sense for ephemrites calculations. It takes the Earth 24hrs to do one full rotation, 24hrs = 360° degrees. Earth takes 12hrs RA to do half a rotation 12hrs RA = 180°. Earth takes 6hrs to do a quarter rotation 6hrs RA = 90°, and so on.

Dec is exactly latitude 0°=above equator 90°=above North Pole, -90°= above South Pole). If you have the RA, Dec of a celestial object and the GPS coordinates of the observer on earth you can calculate the Alt, Az of that object for that observer.

Remember we want Alt=0 and Az=around 53, the GPS location of Chimney Rock is 37°11'30.1"N 107°18'22.9"W so we need to find the RA, Dec that the moon has to be in to have the moon at that Alt, Az at that location then calculate what time the moon will be at that location. The moon’s Dec needs to be maximized to have it in the most northern position possible. The Moon’s orbit around the earth is tilted 5° relative to the ecliptic (ecliptic is the plane of our solar system, if you think of the classic model of the sun and all the planets orbiting it in a flat disk that flat disk is the plane of the solar system or the “ecliptic”). The Earth's “axial tilt” that’s responsible for the seasons is basically the Earth’s tilt to this plane (the ecliptic) too and that is 23°. This means the max Dec the moon is 28° North (5° Moon’s tilt + 23° Earth’s tilt = 28° total tilt with respect to the ecliptic).

Finally there’s the time component, since the moon moves in the sky and the location of the observer is rotating around Earth and that’s all dependent on time. we need to find a time that the moon is rising (Alt=0), and Dec = 28° (max) so that Az can = ~53°. This is where using the trick of stating RA as hours instead of degrees helps because RA doesn't really matter as long as Alt = 0.

This is where a computer can step in. The calculation of ephemrites was actually one of the first applications of mechanical computers. Currently ephemerides are almost purely electronic. You can still find printed ephemerides in old astronomy guide books but anyone that has tried that vs using a phone app to find planets or satellites will have to concede the app is much easier.

I have had my computer run the numbers and found the list of dates and times that the moon will be visible between the rocks this Major Lunar Standstill period and published them in my post here: https://www.astrotours.org/post/how-to-see-major-lunar-standstill-at-chimney-rock-national-monument

If you want to see the Major Lunar Standstill with me contact me https://www.astrotours.org/privatetours.

If you want to learn more about this and all the clockwork of our universe book an AstroTour in Boulder here https://www.astrotours.org/booking

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