Neptune photographed by Voyage. Image credit: NASA/JPL

Happy Anniversary, Neptune!

Article written: 11 Jul , 2011
Updated: 24 Dec , 2015

Today, July 11, 2011 marks the first full orbit of the planet Neptune since its discovery on the night of September 23-24, 1846. But there’s a lot more to learn about this anniversary than just the date. Step inside and let’s find out…

Pinpointing Neptune is a wonderful story. For many years we’ve been taught that the discovery of Neptune was done by mathematical calculations. This came about in 1821 when Alexis Bouvard was publishing his findings for Uranus and noticed a gravitational perturbation. This led him to hypothesize an unknown body was crossing the path. Enter miscommunications, politics and astronomer John Adams…

“It is more likely that Adams realised that his proposed orbits were moving ever closer to a “forbidden” zone of resonance.” says Brian Sheen of Roseland Observatory. “Uranus orbits in 84 years, Neptune in 165, nearly a 2:1 resonance, this brings about much greater perturbations than were being measured. In fact the mid 19th century is a quiet period and much bigger swings are evident now.”

In 1843 John Couch Adams used the data Bouvard proposed to begin working on a proposed orbit, but it would be several years later before Urbain Le Verrier verified its existence through physical observation – at the same time as Johann Gottfried Galle. Says Sheen; “It is often said that Adams never published his results. In fact a published paper was printed by November 1846 and appeared in the 1851 Nautical Almanack published in 1847.”

Unknown to both at the time – and in a great twist of irony – Galileo had actually observed Neptune on December 28, 1612, and again on January 27, 1613, but didn’t realize it was a planet. Small wonder he thought it was a fixed star, because as luck would have it, Neptune turned retrograde at the same time as his first observation! But Galileo was a great observer and made drawings of his find. Given all that we know today, it’s pretty astonishing his limited equipment was able to perceive the blue planet, let alone realize its minor movement against the ecliptic meant something. After all, the very concept of the ecliptic plane was new!

“It has been known for several decades that this unknown star was actually the planet Neptune,” says University of Melbourne physicist, David Jamieson. “Computer simulations show the precision of his observations revealing that Neptune would have looked just like a faint star almost exactly where Galileo observed it.”

But we digress…

Today, July 11 would be the anniversary of Neptune’s first full barycentric orbit – a celebration that has taken us 164.79 years of waiting to celebrate. Tomorrow, July 12 is the anniversary of Neptune’s heliocentric completion. However, don’t expect Neptune to be in the exact same position in relation to the celestial sphere as it was on either date. While over 150 years is but a wink in the cosmic eye, it is certainly more than enough time for our solar system to have shifted That having been over simply said, what will happen at 21:48 and 24.6 seconds UT on July 11 is that Neptune will return to its exact longitudinal position in respect to the invariable plane. Is it close to its discovery point? Well, in a sense, yes. It will be within 1.5 arc seconds of its 1846 location relative to the barycentre. In visual terms, that’s just a whisker.

Is Neptune observable right now? You betcha’. But it’s not going to be easy… You’ll find it at RA 22h 11m 14s – Dec 11 47′ 1″ at its longitudinal anniversary time. Need a map? Here you go…

As you can see, it’s going to be quite late at night before Neptune has well cleared the horizon – but what an opportunity! Because of its small size, I recommend using a telescope for stability and printing a map from a planetarium program for more detailed star fields. It’s certainly not going to look like the Voyager image above, but you can expect to see a slightly blue colored disk that averages about magnitude 8 (well within reach of smaller scopes). If you have never seen Neptune before, compare it in your mind’s eye to one of Jupiter’s moons and you’ll be able to pick it out of starry background much easier.

Good luck, clear skies and happy anniversary Neptune!

Many thanks to Brian Sheen of Roseland Observatory!

13 Responses

  1. Blorch Headblownov says

    Because man is the measure of all things, and Neptune didn’t exist before it was discovered.

    Way to go on re-enforcing scientific ignorance.

  2. Bob Athon says

    Nice article; all lifted from this post, it seems?

    • Member
      Tammy Plotner says

      hi, bob! nice link… i encourage readers to follow it because the gentleman did a much better job than i did at explaining things! here you will find factual information from wikipedia, NASA encyclopedia, sky & telescope, excerpts from brian sheen, a chart from “your sky”, etc. our own Nancy also did a great article that surely helped while writing this one!

  3. Bob Athon says

    Who said it didn’t exist, Blorch? You’re not going to dispell scientific ignorance by denying that its discovery has an anniversary

  4. Mika Viljakainen says

    Search of Planet Vulcan – the Ghost in Newton’s Clockwork Universe by Richard Baum and William Sheehan (1997) gives a nice popular account of why both Adams and Verrier were ‘more wrong than right’, in a hindsight. Accepting fallibility and limitations of one’s methods is important as ever, given how much journalists and publicists hype even flimsiest hypotheses and models, as if everything presented as ‘science’ is standing on the shoulders of giants.

  5. Jesper de Jong says

    Magnitude 8, I guess that’s too faint to see with binoculars? I have 10×50 binoculars and I think the faintest objects I can see from my location (a not very dark sky near a city) is about magnitude 6.

    • Bob Athon says

      It’s clearly visible with my tiny 10x25s on a clear night, even as low in the sky as it is these nights. I’m just a few miles from urban areas. If you can’t make it out from where you are, you shouldn’t have to go far to be able to see it. Definitely worth a look – it’s a real thrill when you know you’ve seen it.

  6. WaNg san says

    I believe thats why they specified “July 11, 2011 marks the first full orbit of the planet Neptune since its discovery on the night of September 23-24, 1846”. They are celebrating the aniversary of observing a full rotation, and considering how long that rotation took from the point of our discovery, I’d say it’s pretty justified.

  7. Duncan Lunan says

    I just did an interview on the anniversary for BBC Radio Scotland. Beforehand I flipped through “The Neptune File” by Tom Standage and found a nice quote (which I didn’t get to use): Jurry van der Woude, Voyager mission team, at the time of the Voyager 2 flyby. “Jupiter is a painted harlot. Neptune is an Audrey Hepburn planet.”

  8. Anonymous says

    I tried to find an article I read back in the 1990s which outlined this history of Neptune’s discovery. It is fairly remarkable that the orbit of this planet was estimated using methods that were not well developed. Classical perturbation theory of Poincare and von Zeiple was developed some decades later towards the end of the 19th century. Bouvard and Le Verrier did some exhausting calculations to pinpoint where to observe.


  9. richard says

    So the Sun and Moon didn’t exist before man came to being. Is that the most ignorant thing I’ve ever heard. So until your old enough to have a real memory of what has happened in your past your Father and Mother didn’t exist.

  10. ITSRUF says

    IIRC there was a good Astronomy Today Podcast about the discovery of Neptune as well. Lots ‘o politics and national pride involved….

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