Image credit: NASA/JPL Cassini Spacecraft.
If you’re a fan of the night sky, you’ve been noticing the trio of Jupiter, Saturn and Mars rising one at a time in the East as it’s been quite a treat for the past few months. If you haven’t seen this yet I would urge you to do so because it’s a fantastic view that’s peaking right now. Beauty aside, they’re also a big trio. Mars hits opposition in a few weeks and Jupiter reached opposition back on May 8. Well now it’s Saturn’s turn as its hitting opposition this Wednesday, June 27, 2018 but the exact date and time doesn’t really matter. Saturn reaches opposition every year or 378 days or 54 weeks and this opposition will bring Saturn to 9 AU (AU being 1 Earth/Sun distance or about 93 million miles) from Earth.
Opposition, in planetary terms is when a superior (outer) planet, dwarf planet or planetary body reaches a point almost exactly opposite the Sun from Earth’s vantage point so we see its full, bright disk. Another way to say it is Earth is positioned directly between the Sun and that object. This occurs because as the outer planets orbit more slowly than the inner planets so every so often we come up on the inside like turn 1 at Laguna Seca, make the pass and we’re on our way until next time. Though Mercury and Venus can never reach opposition as seen from Earth; Earth reaches opposition as seen from them. 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 closest point usually differ from a few hours to a few weeks.
Why opposition doesn’t also 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 no two orbits mirror each other, so even though a planet may be directly opposite the Sun from us, the elliptical nature of orbits usually means that the actual closest point tend to be slightly before or after opposition.
Why oppositions have different distances from opposition to opposition? The reason is the same as the above paragraph; the orbits of the planets aren’t actually circular, they’re slightly elliptical. For example, Mars reaches opposition around the same time it reaches perihelion (closest point to the Sun) every 15 to 17 years so when that happens opposition is closer. Conversely, if Mars reaches opposition at its furthest point from the Sun it will be a more distant opposition than if it were at its closest point. Imagine if we happen to be at aphelion (Earth’s farthest point from the Sun) at the same time say, Mars is at its perihelion and opposition? What a great sight that would be!
Also, during the immediate hours of opposition look for a phenomenon known as the Seeliger Effect; also known as Opposition Surge, Opposition Effect or Opposition Spike. This is a sudden brightening of the planet right before through right after opposition. This also happens with other celestial bodies including Saturn and the Moon. The main culprit for this phenomenon seems to be coherent backscattering and shadow hiding. Using the Moon as an example of shadow hiding; just hours before full moon the suns light is hitting the lunar surface squarely and almost all major shadows from our vantage point are gone. When this occurs the Moon can brighten by about 40 percent and lasts until a few hours after full phase. When this happens during an opposition of Saturn, the rings actually become brighter than the planet itself for those few hours.
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.
Saturn Opposition 2018:
Date of Opposition: Wednesday, June 27, 2018
Time of Opposition: 13:15 UTC (09:15 EDT) though not hugely important
Where is it? (Constellation): Sagittarius
Where is it? (Exact RA/Dec J2000): RA 18h 24m 10s/ Dec -22° 28′ 08”
Apparent Magnitude: A very bright 0.9
Apparent Size (angular size or how large it will be on our night sky): 18.4” arcseconds and including rings it will be 41.7” arcseconds.
Ring Inclination (tilt): 26 degrees
Altitude: About 35 degrees for mid-northern latitudes
Distance from Earth: 9.049 AU (An AU is 1 Earth/Sun distance or 93 million miles)
Opposition Frequency: Just over a year; 54 weeks or 378 days
Moon Phase: Waning gibbous at 99 % illumination
Current visiting spacecraft: None
Former visiting spacecraft: Pioneer 11, Voyager 1, Voyager 2, Cassini
Naked Eye Planets Saturn Location: http://www.nakedeyeplanets.com/saturn.htm
Dominic Ford’s “In-The-Sky” page for this event: https://in-the-sky.org/news.php?id=20180627_12_100
In-The-Sky’s, Solar System Body Finder Chart: https://in-the-sky.org/findercharts.php?startday=1&startmonth=6&startyear=2016&objtype=0&duration=3&objtxt=Saturn&objs=
U.S. Naval Observatory (USNO) Solar System Body Apparent Dimensions: http://aa.usno.navy.mil/data/docs/diskmap.php
U.S. Naval Observatory (USNO) Solar System Body Rise & Set Times: http://aa.usno.navy.mil/data/docs/mrst.php
The Sky Live page for Saturn: http://theskylive.com/saturn-info
The Sky Live, Saturn Tracker: http://theskylive.com/saturn-tracker
The Sky Live, Saturn Planetarium: https://theskylive.com/planetarium?obj=saturn
Chris Peat’s “Heavens-Above” Planet Summary: http://heavens-above.com/PlanetSummary.aspx
The Planets Today (current solar system configuration): http://www.theplanetstoday.com/
Time and Date Sunrise and Sunset calculator: http://www.timeanddate.com/sun/
Time and Date Moonrise and Moonset calculator: http://www.timeanddate.com/moon/