Sky & Telescope - February 2022 - 49

ALAN DYER
Both the winter Milky Way and the zodiacal
light gently glow in this panorama captured at
Dinosaur Provincial Park in Alberta, Canada.
Dust shed by sublimating comets
and colliding asteroids has long been
thought to serve as the source material,
with comets contributing the
bulk. However, in results published in
a March 2021 paper, a team of scientists
working with data collected by
NASA's Juno spacecraft suggest that
much of the zodiacal dust may originate
instead from Mars. As Juno traveled
between Earth and the asteroid belt
en route to Jupiter, one of the probe's
star-tracker cameras serendipitously
recorded minute flashes of light as dust
particles chipped flecks of paint off the
probe's solar arrays. Enough " hits " were
recorded to map the spatial distribution
of the tiny grains.
The group determined that much of
the zodiacal cloud truncates at Earth
because our planet vacuums up the dust
in its vicinity, while Jupiter's gravity acts
as a barrier that keeps the grit at bay
just beyond Mars's orbit. But between
Mars and Jupiter, the dust's orbital
properties closely resemble those of
Mars itself, implicating it as the source.
There's just one problem: No one
has yet figured how the dust escapes
Mars's gravitational pull. And whatever
that process is, it must have been
active for millions of years in order to
replenish the dust lost to Earth and the
Sun. Potential sources include impacts
on the Martian moons Phobos and
Deimos, but again, it's unknown how
the material departs the Martian system.
(You can read the team's paper at
https://is.gd/zodiacal.)
In late winter and spring, the zodiacal
light becomes visible in the western
sky shortly before the end of evening
twilight. During autumn, the phenomenon
appears in the eastern sky at
dawn. It's during these times of year
that the ecliptic lies at a steep angle to
the horizon, tilting the zodiacal light
cone higher into a dark sky and greatly
improving its visibility.
At mid-northern latitudes, astronomical
twilight lasts about 90 minutes
in mid-February. The zodiacal glow
first appears about 20 minutes before
twilight's end and remains in view for
up to several hours. Peak brightness and
maximum extent occur right at the end
of evening twilight in spring, and just
before the start of morning twilight in
fall. You don't need perfect skies, but
you'll want to avoid moonlight. Upcoming
Moon-free observing windows run
from January 20th to February 2nd;
February 18th to March 3rd; and from
March 19th to April 3rd.
Two Lunar Occultations
Few sights ping the optic nerve like seeing a star suddenly disappear or
reappear along the Moon's limb. It's a startling experience and illustrates
just how devoid the Moon is of air - an atmosphere would cause the star to
fade gradually instead of instantly wink out. Occultation timings are also
used to measure stellar diameters and separate extremely close double and
multiple stars.
On February 9th, the dark limb of the 65%-illuminated waxing gibbous
Moon occults both Kappa1 and Kappa2 (κ1 and κ 2) Tauri. The Kappa pair are
separated by 5.7′ and shine at magnitudes 4.2 and 5.3, respectively. In the
middle of North America, the event begins round 10:30 p.m. CST, though the
time will vary considerably depending on your location. The occultation will
be visible across most of the U.S. and Canada, but the double event excludes
the far southern U.S., where the Moon only eclipses Kappa1. For locations
where the Moon passes centrally over the pair, the back-to-back disappearances
occur just seconds apart.
A second noteworthy occultation occurs on the morning of February 22nd
as the 66% waning gibbous Moon covers the double star Alpha (α) Librae,
better known as Zubenelgenubi. The two Alpha components are a generous
3.9′ apart and twinkle at magnitude 5.2 (Alpha1) and 2.7 (Alpha2). The
Zubenelgenubi eclipse will be visible for observers across the western threequarters
of the U.S., western Canada, Mexico, and parts of Central America.
The bright limb of the Moon first covers dim Alpha1 - a part of the show that
may be tricky to see. The disappearance of Alpha2, however, should be much
easier to catch since it's considerably brighter. The potentially more dramatic
moment happens when the stars pop out in rapid succession from behind the
dark lunar limb.
The show starts around 5:45 a.m. CST, with the stars reappearing around
6:30 a.m. Once again, the exact time depends on where you're situated.
For maps and up-to-date times for a variety of different cities for both
events, consult the International Occultation Timing Association's website:
lunar-occultations.com/iota/iotandx.htm.
sk yand tele scope .o r g * FEBRUARY 2022 49
https://www.is.gd/zodiacal http://www.lunar-occultations.com/iota/iotandx.htm
Sky & Telescope - February 2022

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