November 20, 2018

Astro-Challenge: Spot Neptune in its Original Discovery Position!

In this week’s astro-event, we challenge you, the sky watching public, to view the planet Neptune as it was first seen on the night of its discovery on September 23, 1846. On that evening, astronomer Johann Galle turned the Berlin Observatories’ 9-inch refractor on a position given to him by French mathematician Urbain Jean-Joseph Le Verrier, and the solar system hasn’t been the same since. The discovery of Neptune was a triumph for predictive mathematics and a good test of Newtonian mechanics in a celestial format.

The good news is, Neptune is about to complete one 164.8 year orbit in one year’s time on July 12th, 2011. You don’t, however, have to wait that long to see the outermost gas giant as Galle did. In fact, due to its retrograde looping caused by our shifting vantage point here on planet Earth, three more sighting ops to catch Neptune on the Aquarius-Capricornus border exist; November 22, 2011; October 27, 2011; and this Saturday, on July 17th, 2010. Sighting ops will be best in the AM predawn hours, as Neptune rises just past 11 PM local on the 17th and transits just before 5 AM. The Moon will be approaching 1st Quarter and thus will set just about when Neptune is rising… the planet will show a tiny, grayish-blue disk about 2.3” arc seconds in size at about a +7.9 magnitudes in brightness. Good luck, and be sure to maintain join in this dawn vigil that has its roots in astronomical history!

This week’s Astro-word is Perturbation. This is the gravitational pull of one astronomical body on another. All bodies in the solar system exert a pull on other bodies to some degree, altering orbits ever so slightly. Over millions of years, these tiny effects do add up, causing changes in eccentricities and other orbital elements. Because of this complexity, predicting the exact position of astronomical bodies gets more imprecise the farther you look ahead in time. Gravitational perturbation played a key role in the discovery of Neptune, as astronomers soon realized that the new found planet Uranus wasn’t exactly where it should be. Of course, those astronomers of yore were also fortunate that Uranus and Neptune were within 45° degrees of solar longitude of each other around the time of 1846 when this effect was the greatest; otherwise, the discovery of Neptune might have had to wait another century! Of course, Galileo glimped (and sketched!) the planet Neptune during a conjunction with Jupiter, but that’s another tale…


  1. [...] Astro-Challenge: Spot Neptune in its Original Discovery Position … [...]

  2. [...] conjunction of Neptune and Uranus occurred back in 1993, and won’t occur again until 2164. Heck, In 2010, Neptune completed its first orbit since [...]

  3. [...] also sobering to think that Neptune has only recently completed a single orbit of the Sun in 2011 since its discovery. Opposition of Neptune occurs once every 368 days, meaning [...]

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