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Earth at Aphelion
Published: 4 Jul , 2016
by David Dickinson
Having a great July 4th? The day gives us another cause to celebrate, as the Earth reaches aphelion
today, or our closest point to our host star.
Aphelion is the opposite of the closest point of the year, known as perihelion. Note that the ‘helion’ part
only applies to things in solar orbit, perigee/apogee for orbit ’round the Earth, apolune/perilune for
orbit around the Moon, and so on. You’ll hear the words apijove and perijove bandied about this week a
bit, as NASA’s Juno spacecraft enters orbit around Jupiter tonight. And there are crazier and even more
obscure counterparts out there, such as peribothron and apobothron (orbiting a black hole) and
apastron/periastron (orbiting a star other than our Sun). And finally, there’s the one-size fits all generic
periapsis and apoapsis, good for all occasions and ending pedantic arguments.
In the 21st century, aphelion for the Earth can actually fall anywhere from July 2nd to the 7th. The once
every four year leap day is the primary driver in this oscillation, and the inclusion of a century leap day
in 2100 — the first since 1900 — will reset things even farther astray.
Aphelion versus perihelion. (orbits exaggerated). Image credit: NOAA/NASA.
In 2016, the Moon reaches New on July 4th at 11:01 Universal Time (UT) just over five hours prior to
aphelion, marking the start of lunation 1157. The sighting of the waxing crescent Moon also marks the
end of the Muslim fasting month of Ramadan.
Earth reaches aphelion at 16:24 UT today, 1.0168 AU from the Sun. This year’s close occurrence of
aphelion versus a New Moon won’t get topped until 2054, with an aphelion versus New Moon just 5
hours 6 minutes apart. The 2016 coincidence is also the closest since the start of the 21st century.
Fun fact: we’re headed towards an aphelion maximum just 6,590 kilometers off of the mean on July
4th, 2019, the widest for the 21st century. Mean distance from the Sun at aphelion is 1.0167 AU
(152,097,701 km). Aphelion for the Earth can range over a variation of 21,225 kilometers for the 21st
century.
https://www.youtube.com/watch?v=Rsp4-m_gMz8
It’s a happy circumstance that Earth reaches aphelion in our current epoch in the midst of northern
hemisphere summer, and just a few weeks after the June solstice. The eccentricity of the Earth’s orbit
actually varies from near-circular to 0.0679 and back over the span of 413,000 years. In our current
epoch, the eccentricity of our orbit is 0.017 and decreasing. Add this variation to changes in the axial
tilt of our planet and orbital obliquity, and you have what are known as Milankovitch Cycles. One only
has to look at Mars’s wacky orbit with an eccentricity of 0.0934 to see what a difference it makes.
Ironically, Mars reaches perihelion in October 29th, 2016, and will make a very close pass near next
opposition pass in 2018.
The orbits of Mercury, Earth and Mars compared. image credit: NASA
Want to prove it for yourself? You can indeed ‘observe’ aphelion. The trick is to image the solar disk
using the same rig and settings… about six months apart. At aphelion, the solar disk is 31.6′ across,
versus 32.7′ across at perihelion. This variation is slight, but you can indeed see the subtle difference
side by side:
The Sun as seen from the Earth: perihelion vs aphelion. The red circles are the size of the opposing
solar disk transposed on the other. Image credit: Dave Dickinson.
Aphelion means a smaller apparent Sun, a good target for a total solar eclipse. Stick around until July
2nd, 2019 and you’ll see just that, as a total solar eclipse occurs near aphelion for South America and
the southern Pacific at 4 minutes and 33 seconds in central duration.
This month also sees another special treat, as all classical planets enter the evening sky.
More to come on that soon. For now, happy 4th of July, and merry aphelion!
Related
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Catching Earth at Aphelion
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Orbits of the Planets
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By David Dickinson David Dickinson is an Earth science teacher, freelance science writer, retired USAF veteran &
backyard astronomer. He currently writes and ponders the universe as he travels the world with his
wife.