May 28, 2020

AstroEvent: The Many Birthdays of Neptune.

Current position of Neptune versus discovery. (Created by Author using Starry Night).

Mark your calendars; the planet Neptune, the first world discovered by a tour de force of pure mathematics, passes a landmark this week…sort of. The story harkens way back to a prediction made by Urbain Le Verrier of the position of an object tugging on the Planet Uranus. (Yes, we know of the John Couch Adams controversy!) The object was spotted by Johann Galle and Heinrich d’Arrest at the Berlin observatory on the night of September 23-24th 1846 using Le Verrier’s predictions and was within a degree of where it should be. But did you know that Neptune is just now completing its first full orbit since discovery?

Wait a minute, I hear a faithful reader out there saying to himself, didn’t we celebrate Neptune returning to its discovery position last year? In fact, we did. Neptune takes 164.79 Earth years to complete one orbit; it’s been zigzagging past that original discovery position of about R.A. 21hrs 53’ 17” Declination -13° 24’ 08” once on July 17, 2010, and twice more on October 27th & November 22nd of this year…  so why isn’t Neptune precisely in the same position this month? The primary culprit is parallax. Remember, the planet Neptune is about 30 Astronomical Units distant; that means it can show an apparent positional shift of about 3° from either side of the Earth’s orbit. We’re not in the same position as Galle and D’Arrest where that fateful night is September 1846. Complicating matters are such factors as the eccentricity of the orbits of our two respective worlds, plus the 1.7° degree tilt of Neptune’s orbit relative to the ecliptic… but wait, there’s more. The coordinates quoted above for the discovery where epoch 1850.0… currently, Neptune is very near R.A. 22 Hours 11’ 30” and a declination of -11° 45’ 24” in 2000.0 coordinates! Still, by some reckonings, July 12th may be as good a date as any… Neptune approaches opposition this year on August 22nd and currently rises around 11PM local… do give this distant 8th magnitude world a look as it completes a full circuit after discovery!

The Astro Word of the Week is: Solar Longitude. This is simply a method of marking a position along the ecliptic, which in turn is an imaginary line drawn on the celestial sphere that is a reflection of the Earth’s orbit. Of course, from our vantage point, it’s the Sun that traces out the path of the ecliptic during the course of a terrestrial year. We mark the position of solar longitude as 0° starting at the vernal equinox, or one of the two points that the ecliptic and the celestial equator intersect each other. This brings Neptune back to a solar longitude very near 329° 05’ 33” this summer assuming a sun-centered barycenter for the solar plane (remember, among other things, the 26,000 year precession of the equinoxes has moved our own plane of reference 2.2° in 164+ years! Such are the convoluted motions of our respective worlds about the Sun; stay tuned towards said Neptunian opposition next month as we go faint moon-spotting!


  1. [...] light bucket you’ve got sitting in your backyard to good use. August 22nd, the planet Neptune reaches opposition. And yes, we know you’ve spotted the grey-blue world at magnitude +7.9 [...]

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