Observing Neptune: A Guide to the 2014 Opposition Season


Never seen Neptune? Now is a good time to try, as the outermost ice giant world reaches opposition this weekend at 14:00 Universal Time (UT) or 10:00 AM EDT on Friday, August 29th. This means that the distant world lies “opposite” to the Sun as seen from our Earthly perspective and rises to the east as the Sun sets to the west, riding high in the sky across the local meridian near midnight.

2014 finds Neptune shining at magnitude +7.6 in the constellation of Aquarius. Unfortunately, the planet is too faint to be seen with the naked eye, but can be sighted using a good pair of binoculars if know exactly where to look for it. Though the telescope, Neptune exhibits a tiny blue-gray disk 2.4” across — 750 “Neptunes” would fit across the apparent diameter of the Full Moon — that’s barely discernible. Don’t be afraid to crank up the magnification in your quest. We’ve found Neptune on years previous by patently examining suspect stars one by one, looking for the one in the field that stubbornly refuses to focus to a star-like point. Make sure your optics are well collimated to attempt this trick. Neptune will exhibit a tiny fuzzy disk, much like a second-rate planetary nebula. In fact, this is where “planetaries” get their moniker, as the pesky deep sky objects resembled planets in those telescopes of yore…

Looking eastward
The position of Neptune, looking eastward on the night of opposition around an hour after sunset. Created using Stellarium.

The 1846 discovery of Neptune stood as a vindication of the (then) new-fangled theory of Newtonian gravitational dynamics. Uranus was discovered just decades before by Sir William Hershel in 1781, and it stubbornly refused to follow predictions concerning its position. French astronomer Urbain Le Verrier correctly assumed that an unseen body was tugging on Uranus, predicted the position of the suspect object in the sky, and the race was on. On the night of September 24th, Heinrich Louis d’Arrest and Johann Gottfried Galle observing from the Berlin observatory became the first humans to gaze upon the new world referring to it as such. Did you know: Galileo actually sketched Neptune near Jupiter in 1612? And those early 18th century astronomers got a lucky break… had Neptune happened to have been opposite to Uranus in its orbit, it might’ve eluded discovery for decades to come!

It’s 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 that opposition is slowly moving forward by about three days a year on our Gregorian calendar and will soon start occurring in northern hemisphere Fall.

September 15th
Neptune and a one degree field (green) circle. Note that it passes the bright naked eye star Sigma Aquarii on September 15th. Created using Starry Night Education Software.

Now for the “wow factor” of what you’re actually seeing. Though tiny, Neptune is actually 24,622 kilometres in radius, and is 58 times as big as the Earth in volume and over 17 times as massive. Neptune is 29 A.U.s or 4.3 billion kilometres from Earth at opposition, meaning the light we see took almost four hours to transit from Neptune to your backyard.

Neptune is currently south of the equator, and won’t be north of it again until 2027.

Next month, keep an eye on Neptune as it passes less than half a degree north of the +4.8 magnitude star Sigma Aquarii through mid-September, making a great guide to find the planet…

Aug 29
The orbit of Triton on the evening of August 29th, superimposed on a one arc minute field of view. Created using Starry Night Software.

Still not enough of a challenge? Try tracking down Neptune’s large moon, Triton. Orbiting the planet in a retrograde path once every 5.9 days, Triton is within reach of a large backyard scope at magnitude +14. Triton never strays more than 15” from the disk of Neptune, but opposition is a great time to cross this curious moon off of your observing life list. Neptune has 14 moons at last count.

And speaking of Triton, NASA recently released a new map of the moon. We’ve only gotten one good look at Triton, Neptune, and its retinue of moons back in 1989 when Voyager 2 conducted the only flyby of the planet to date.  Will Pluto turn out to be Triton’s twin when New Horizons completes its historic flyby next summer?

The Moon also passes 4.3 degrees north of Neptune on September 8th on its way to “Supermoon 3 of 3” for 2014 on the night of September 8th/9th. Fun fact: a cycle of occultations of Neptune by the Moon commences on June 2016.

When will we explore Neptune once more? Will a dedicated “Neptune orbiter” ever make its way to the planet in our lifetimes? All fun things to ponder as you check out the first planet discovered using scientific reasoning this weekend.

How to See Planet Neptune: Our Guide to Its 2013 Opposition

Neptune and its large moon Triton as seen by Voyager 2 on August 28th, 1989. (Credit: NASA).

If you do your own stargazing or participate in our Sunday night Virtual Star Parties, you’ve probably noticed we’re starting to lose planetary targets in the night-time sky. August and September of this year sees Venus and Saturn to the west at dusk, with the planets Mars and Jupiter adorning the eastern dawn sky just hours before sunrise.

That means there is now a good span of the night that none of the classic naked eye planets are above the horizon. But the good news is, with a little persistence, YOU can spy the outermost planet in our solar system in the coming weeks: the elusive Neptune. (Sorry, Pluto!)

A wide field view of Aquarius with Neptune's present position in late August/early September. (Created by the author i Stellarium).
A wide field view of Aquarius, with Neptune’s present position marked for late August/early September. (Created by the author in Stellarium; click on all star charts to enlarge).

The planet Neptune reaches opposition late this month in the constellation Aquarius on August 27th at 01:00 UT (9:00 PM EDT on the 26th). This means that it will rise to the east as the Sun sets to the west and will remain above the local horizon for the entire night.

If you’ve never caught sight of Neptune, these next few weeks are a great time to try. The Moon passes 6° north of the planet’s location this week on August 21st, just 10 hours after reaching Full.

neptune on the night of Opposition using a 4.4 degree field of view. (Created by the author using Stellarium).
Neptune on the night of Opposition using a 4.4 degree field of view. Magnitudes for nearby guide stars are given in red. (Created by the author using Stellarium).

Shining at magnitude +7.8, Neptune is an easy catch with binoculars from a dark sky site. Even in a large telescope, Neptune appears as a tiny blue dot, almost looking like a dim planetary nebula that refuses to come to a sharp focus. Visually, Neptune is only 2.3” across at opposition; you could stack 782 Neptunes across the breadth of the Full Moon!

It’s sobering to think that Neptune only just returned in 2011 to the position of its original discovery back in 1846. The calculation of Neptune’s position by Urbain Le Verrier was a triumph for Newtonian mechanics, a moment where the science of astronomy began to demonstrate its predictive power.

Astronomers knew of the existence of an unseen body due to the perturbations of the planet Uranus, which was discovered surreptitiously by William Herschel 65 years earlier. Using Le Verrier’s calculations, Johann Galle and Heinrich d’Arrest spied the planet on the night of September 23rd, 1846 using the Berlin observatory’s 9.6” refractor. Neptune was within a degree of the position described in Le Verrier’s prediction.

Neptune orbits the Sun once every 164.8 years, and comes back into opposition once every successive calendar year about 2 days later than the last. Those observers of yore were lucky that Neptune and Uranus experienced a close and undocumented conjunction in 1821; otherwise, Neptune may have gone undetected for a much longer span of time. And ironically, Galileo sketched the motion of Neptune near Jupiter in 1612 and 1613, but failed to identify it as a planet!

The motion of Neptune through Aquarius through late August into September. (Starry Night)
The motion of Neptune through Aquarius through late August into September. (Graphic created by author; courtesy of  Starry Night Education).

Neptune descended through the ecliptic in 2003 and won’t reach its southernmost point below it until 2045. This month, Neptune lies 1.5 degrees west of the +4.8 magnitude double star Sigma Aquarii. Neptune passes less than 4’ from +7.5 magnitude star HIP 110439 on September 9th as it continues towards eastern quadrature on November 24th.

Corkscrew chart credit:
Corkscrew chart for the elongations of Triton through early September. (Courtesy of Ed Kotapish; created using NASA/PDS Rings Node).

Up for a challenge? Neptune also has a large moon named Triton that is just within range of a moderate (8” in aperture or larger) telescope. Shining at magnitude +13.4, Triton is similar in brightness to Pluto and is 100 times fainter than Neptune. In fact, there’s some thought that Pluto may turn out to be similar to Triton in appearance when New Horizons gets a close-up look at it in July 2015.

Triton never strays more than 18” from Neptune during eastern or western elongations. This presents the best time to cross the moon off your astronomical “life list…” experienced amateurs have even managed to image Triton!

The moons of Neptune and Uranus imaged by Credit:
The moons of Neptune and Uranus imaged by Rolf Wahl Olsen using a 10″ reflector and a ToUCam Pro. An amazing catch! (Credit & Copyright: Rolf Wahl Olsen, used with permission).

Triton was discovered just 17 days after Neptune by William Lassell using a 24” reflector. Triton is also an oddball among large moons in the solar system in that it’s in a retrograde orbit.

A second moon named Nereid was discovered by Gerard Kuiper in 1949. To date, Neptune has 14 moons, including the recently discovered S/2004 N1 unearthed in Hubble archival data.

Neptune & Triton on the night of August 21st as it reaches greatest elongation. (Starry Night).
Neptune & Triton on the night of August 21st as it reaches greatest elongation. (Graphic created by author. Courtesy of Starry Night Education).

To date, Voyager 2 is the only spacecraft that has studied Neptune and its moons up close. Voyager 2 conducted a flyby of the planet in 1989. A future mission to Neptune would face the same dilemma as New Horizons: a speedy journey would still take nearly a decade to complete, which would rule out an orbital insertion around the planet. (Darn you, orbital mechanics!) In fact, New Horizons just crosses the orbit of Neptune at a distance of 30 astronomical units from the Earth in 2014.

Neptune is about four light hours away from the Earth, a distance that varies less than 20 minutes in light travel time from solar conjunction to opposition. And while Neptune and Triton may not appear like much more than dim dots through a telescope, what you’re seeing is an ice giant 3.8 times the diameter of the Earth, with a large moon 78% the size of our own.

Make sure to cross Neptune and Triton off of your bucket list… and next month, we’ll be able to do the same for the upcoming opposition of Uranus!