Universe Today
Having a great July 4
th
? The day gives us another cause to celebrate, as the
Earth reaches aphelion
today, or our farthest 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 21stcentury
, 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 exclusion of a century leap day in 2100 — the first such exclusion since 1900 — will reset things even farther astray.
[caption id="attachment_129711" align="aligncenter" width="580"]
Aphelion versus perihelion. (orbits exaggerated). Image credit: NOAA/NASA.[/caption]
In 2016, the Moon reaches New on July 4
th
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 21
st
century.
Fun fact: we're headed towards an aphelion maximum just 6,590 kilometers off of the mean on July 4
th
, 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 21
st
century.
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 29
th
, 2016, and will make a very close pass near next opposition pass in 2018.
[caption id="attachment_129710" align="aligncenter" width="312"]
The orbits of Mercury, Earth and Mars compared. image credit: NASA[/caption]
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:
[caption id="attachment_129712" align="aligncenter" width="580"]
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.[/caption]
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.
[caption id="attachment_129715" align="aligncenter" width="580"]
Aphelion versus perihelion 2008. Image credit and copyright:
Alex Conu
.[/caption]
This month also sees another special treat, as all classical planets enter the evening sky.
More to come on that soon. For now, happy 4
th
of July, and merry aphelion!
