Now is the Time to Observe Uranus

Image credit & copyright: NASA/JPL Voyager 2.

Thursday, October 19, 2017 at 13:21 EDT (17:21 UTC) the ice giant, Uranus will reach opposition which means now is the time to observe the seventh planet. Located in the constellation Pisces, the 15, 759 mi. (25,361 km) diameter green/blue sphere is directly opposite the Sun from our vantage point here on Earth which gives us a full disk view while being placed at its closest point to earth (roughly). As this event falls on new moon night, the 5.7 magnitude Uranus will be within naked eye visibility range which is a relatively rare treat.

Odd Uranus Facts: When William Herschel discovered Uranus in 1781 he named it George or Georgium Sidus for King George III but it was later changes to the Greek God of the Sky, Uranus. Uranus also orbits on its side so it sort of “rolls” around the Sun vs. a normal “counter clockwise” rotation.  The two other exceptions are Venus which rotates backward (clockwise) and Neptune’s moon, Triton which orbits the planet backward (clockwise).

NASA solar system illustration of opposition vs closest approach using Mars 2016 opposition phase.

What is Opposition?:

Opposition, in planetary terms is when a superior (outer) planet, dwarf planet or asteroid reaches a point almost exactly opposite the Sun from Earth’s vantage point so we see its full, bright disk and it will act in a similar fashion as the full moon as it will rise at sunset, reach its highest point around midnight and set around sunrise.Another way to say it is Earth is directly between the Sun and said object.  Opposition also places the object near its closest point that it can get to the Earth in its orbit (perigee). Technically, opposition and the actual perigee points usually differ from a few hours to a few weeks but again, its close enough for what we’re doing.

Why opposition doesn’t always equal perigee or that body’s closest point to Earth.  Well, as I said above it pretty much does but you have to remember that orbits are elliptical and two orbits don’t mirror each other so even though a planet may be directly opposite the Sun from us, the elliptical nature of orbits usually means the actual closest points tend to be slightly before or after opposition.

Why do oppositions have different distances opposition to opposition? The reason is actually the same as in the above paragraph; the orbits of the planets aren’t actually circular they’re slightly elliptical. For example; if Mars reaches opposition during perihelion while Earth happens to be at aphelion that would place Mars in quite a position on our night sky. But if the opposite were true, Mars at aphelion and Earth at perihelion during Mars opposition it wouldn’t be as impressive.

Also, this won’t apply to Uranus but it does for planetary bodies like Saturn and the Moon. During the precise hours of opposition, look for a phenomenon known as the Seeliger Effect. You may also know this as “Opposition Surge”, “Opposition Effect” or even “Opposition Spike.” This is a sudden brightening of the planet (especially Saturn’s rings) just before and just after opposition. For Saturn you may notice that the disk of the planet doesn’t brighten; just the rings. The brightening is from the sunlight being reflected off the ice crystals in the rings as they are illuminated from almost the same direction in which we are viewing them from. With other celestial bodies including the Moon this happens for slightly different reasons that include shadows etc.

Below I’ve listed the important data as well as a bunch of my favorite links to use and abuse as they’re invaluable assets for sky watching and as always, if you have questions, please ask!  I don’t include Planet, Sun or Moon rise and set times because that’s different depending on where exactly you are but the resources are in the links below.

Just the Facts:

Opposition time & date: Thursday, October 19, 2017 at 13:21 EDT (17:21 UTC) but the exact time isn’t important.  The surrounding days to weeks will have it well placed.

Opposition object: Uranus, the 7th planet.

How far away is it: 18.9 astronomical units (AU) or nearly 19 times the distance from the Earth to the Sun.

How big is it? (actual & apparent sizes): 15, 759 mi. (25,361 km) diameter and will be 3.7” arcseconds on the night sky.

Where is it? (constellation): Constellation Pisces.

Where is it? (Exact RA & Dec J2000): R.A: 01h 38m 26s   Dec: +09° 35’ 24”.

Opposition frequency: Roughly every 369.6 days or about 4 days later each successive year.

Apparent magnitude: 5.7 which is within naked eye range under dark skies.

Moon phase & interference: New Moon which means no impact at all.

Dominic Ford’s “In The Sky” page for this event: https://in-the-sky.org/news.php?id=20171019_12_100

Planet locations “The Sky Live”: https://theskylive.com/planets

Planet rise and set times “U.S. Naval Observatory”: http://aa.usno.navy.mil/data/docs/mrst.php

Planet rise and set times “Time and Date”: https://www.timeanddate.com/astronomy/night/usa/boston

Planet rise and set times “Heavens Above”: http://heavens-above.com/planetsummary.aspx?lat=33.647&lng=-117.688&loc=Lake+Forest&alt=148&tz=PST

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