52 ASTRONOMY • OCTOBER 2019
Cross Uranus and Neptune off your
bucket list this month — and then go
for their moons. BY MICHAEL E. BAKICH
Observe the
ice giants
FOR UNTOLD THOUSANDS OF
YEARS, humanity knew of only five
planets. But on March 13, 1781, Friedrich
Wilhelm Herschel discovered what he
initially thought was a comet, altering
the celestial landscape.
Soon after his discovery, astronomers
realized that Herschel had discovered the
first new planet since antiquity: Uranus.
The astronomical community was
delighted. With one observation through
a 6½-inch ref lector, Herschel had dou-
bled the radius of the solar system.
In 1820, French astronomer Alexis
Bouvard compiled position tables for
Uranus. He set out to define, from obser-
vations made since its discovery, the plan-
et’s orbit. But the orbit he deduced didn’t
match either pre-discovery positions of
Uranus (when it had been spotted without
knowing its true nature) or those made
after its discovery. And its deviation from
the older ones was striking.
Urbain Jean Joseph Le Verrier, an
astronomer at the Paris Observatory,reduced the problem to a single question:
“Can the anomaly be explained by the
supposed action of a foreign and hitherto
unknown body on Uranus?” After lengthy
calculations, he sent the presumed object’s
position to several observatories. At the
Berlin Observatory, Johann Gottfried
Galle discovered Neptune on the day he
received the letter from Le Verrier —
September 23, 1846 — less than 1° from
the predicted position.
Astronomers call these two worlds the
ice giants. This term refers to their com-
positions and distinguishes them from
the gas giants Jupiter and Saturn, which
are mainly hydrogen. Uranus and
Neptune contain much less hydrogen
and feature other elements, such as car-
bon, nitrogen, oxygen, and sulfur.
Uranus, which lies an average of
1.8 billion miles (2.9 billion kilometers)
from the Sun, and Neptune, 2.8 billion
miles (4.5 billion km) away, are on every
amateur astronomer’s “life list.” Just see-
ing the ice giants through binoculars ora small telescope is a score for beginners.
But what can you see if you look care-
fully and through a medium or large
scope? This is the month to find out.Where to look
Uranus reaches opposition October 28.
When an outer planet is at opposition, it
lies on the side of Earth opposite the Sun.
It rises at sunset, stands highest in the
south at midnight (which may be 1 a.m.
if Daylight Saving Time is in effect),
and sets at sunrise. The planet also is
closest to Earth around opposition, so
its apparent size is largest. Finally, the
Sun is behind us, so the planet’s entire
sunlit side is in view. Add these factors
together, and you’ll get a planet that’s at
its brightest. Uranus will reach magni-
tude 5.7 on the 28th, when its disk will
measure 3.7" across.
Neptune reached opposition
September 10, when it shone at magni-
tude 7.8 and its disk was 2.4" in diameter.
However, Neptune’s brightness doesn’t
vary much, so it will only be a few hun-
dredths of a magnitude fainter by the
time Uranus reaches opposition.
The orbit of Uranus tilts less than 1°
from the plane of the ecliptic, so you’ll
always find it close to that line. Uranus’
average apparent motion (against the
background stars) is approximately 42"
per day. It takes Uranus about 44 days to
move the width of the Full Moon.
Uranus currently lies in front of the stars
of Aries the Ram. To find it, look 10.5°
south-southeast of magnitude 2.0 Hamal
(Alpha [α] Arietis).ABOVE: Uranus, named for the
Greek god of the sky, orbits the
Sun once every 84 years and
7.5 days. It has 63 times the
volume of Earth and 14.5 times
our planet’s mass. Uranus’ axis
tilts 97.77°, so, in effect, it rolls
around the Sun on its side. NASALEFT: This amateur image of
Uranus, taken October 13, 2017,
through a 12½-inch Newtonian
reflector, shows the planet and
four of its five largest moons.
MARC DELCROIXOberonArielUmbrielTitania