Go the distance: Neptune’s opposition amid a busy week for the planet


Voyager 2’s view of Neptune’s south pole as it passed by the planet 25 years ago. (NASA)

Neptune – the most-distant official planet in our solar system – just celebrated the silver anniversary of the Voyager 2 flyby, and felt a far-away whoosh from another spacecraft soaring past. The planet reaches what astronomers call “opposition” on Friday.

This frigid planet is the eighth from sun and the fourth largest planet in our solar system. Neptune is having a busy week.

The planet Neptune. (NASA) The planet Neptune. (NASA)

In astronomical terms, opposition simply means that the planet is opposite to the sun, from Earth’s perspective. The sun, Earth and Neptune form a straight line. Think of it as a “full” Neptune, like you would think of a full moon. (When the moon is full, it is opposite the sun from our earthly vantage point.)

Neptune has a 164-year orbit around the sun, so it is generally in the same place it was back in 1850.

If you have a telescope or a mounted, steady pair of binoculars, you may catch a glimpse of the cold, distant planet, as Neptune rises at 7:38 p.m. in the east-southeast heavens on Friday, according to the U.S. Naval Observatory. (The sun sets in the west at 7:42 p.m.) Neptune loiters in the Aquarius constellation, snuggling near the fourth-magnitude star Sigma Aquarii, says Geoff Chester, an astronomer with the Naval Observatory. Neptune’s 7.8 magnitude sits far outside naked-eye visibility range. It is due south at 1:12 a.m. on August 30, and sets at 6:41 a.m. in the west-southwest.

Each day, Neptune’s rise and set times vary by a few minutes. Use the Naval Observatory’s free online service for determining these times for major solar system bodies.


The view of Neptune and one of its fourteen moons, Triton (small crescent), from Voyager 2 as it passed by the planet 25 years ago. (NASA)

If you are without a telescope or binoculars, check Slooh for a live view of Neptune from the Institute of Astrophysics, Canary Islands, starting at 8 p.m. ET on Friday. The four-hour image stream will be accompanied by astronomical discussions, including Slooh astronomer Bob Berman.

On August 25, 1989, the Voyager 2 spacecraft flew by Neptune, offering the first close-up views of the far-away world. On Monday, the New Horizons spacecraft crossed Neptune’s orbit at about 2.5 billion miles away – or about 4 billion kilometers away from Earth, according to NASA. For the New Horizons spacecraft, which was launched in January 2006, this is the last major event until its close encounter with the dwarf planet Pluto on July 14, 2015.

Unlike the other planets and how they were discovered, Neptune was found by mathematical reasoning. Using French mathematician Urbain Le Verrier’s calculations, Johann Gottfried Galle of the Berlin Observatory found the distant planet on September 23, 1846. In addition to Le Verrier, credit is often given to English mathematician and astronomer John Couch Adams, as he performed similar calculations concurrently.

Early in the history of the Naval Observatory, Matthew Fontaine Maury was its superintendent at the time of the discovery. Maury had astronomer Sears Cook Walker attempt to find Neptune, based on the astonishing news from Europe. Using the data, Walker determined that the planet had been seen before and record by French astronomer Jermone Lalande in 1795, according to the historian Steven J. Dick’s book, “Sky and Ocean Joined.” Because Neptune had gone into retrograde motion, the planet appeared stationary. Lalande thought he was noting a distant star.

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Angela Fritz · August 27, 2014

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