RECENT AND UPCOMING
PLANET-PLANET OCCULTATIONSDate Foreground planet Background planet
December 9, 1808 Mercury Saturn
January 3, 1818 Venus Jupiter
November 22, 2065 Venus Jupiter
July 15, 2067 Mercury Neptune
August 11, 2079 Mercury Mars
October 27, 2088 Mercury Jupiter
April 7, 2094 Mercury JupiterWWW.ASTRONOMY.COM 69SEND US YOUR
QUESTIONS
Send your
astronomy questions
via email to askastro@
astronomy.com, or
write to Ask Astro,
P.O. Box 1612,
Waukesha, WI 53187.
Be sure to tell us
your full name and
where you live.
Unfortunately, we
cannot answer all
questions submitted.It’s rare — but not impossible — for two planets in the
solar system to line up such that the nearer one occults
the other. This is because although the planets orbit the
Sun in roughly the same plane, their orbits are all slightly
tilted with respect to one another, so two planets rarely
appear to be in the same place at the same time.
Astronomer Steven Albers calculated how many
“mutual occultations” visible from Earth have occurred
or will occur between the years 1557 and 2230. In total, he
deter m i ned t here were 21 occu ltat ions i n t hat t i me f ra me
— an average of about one occultation every 33 years.
However, some centuries have more than one occultation,
while others have none; for example, Albers found no
mutual occultations in the 20th century, but two occulta-
tions in the 19th century and five in the 21st century.
It ’s a l so wor t h not i ng t hat ma ny mut u a l occ u lt at ions
are difficult or impossible to view because they are vis-
ible only from the ocean or occur when the planets are
close to the Sun in the sky, which washes out their light.
In some historical cases, occultations may have been
seen but not recorded as the passage of one planet in
front of another because one of the planets involved,
such as Uranus or Neptune, had not yet been discovered
and was instead labeled as a star.
Alison Klesman
Associate Editor
QI
IF 85 PERCENT OF THE MASS IN
THE UNIVERSE IS DARK MATTER,WHY DOESN’T IT OBSCURE LIGHT FROM
DISTANT OBJECTS? HOW COULD THIS
MATERIAL NOT SCATTER LIGHT?
Jerome Handley
Turlock , California
AI
Da rk mat ter is c a l led “ d a rk ” bec au se it doe sn’t
give off or interact with light — including
through scattering. It is simply the nature of dark matter
and why it is so difficult to study. But some models of
dark matter state that on rare occasions, dark matter
particles could be capable of interacting with normal
matter, including by scattering light.
Astronomers know that dark matter is largely situ-
ated in spherical halos that enclose galaxies (more on
that in a moment). If the dark matter in that halo scat-
ters the galaxy’s starlight, even rarely, it could create a
dim glow, like the halo of a light seen in thick fog.
Researchers have searched for that glow but so far have
not seen it. One possibility is that the glow is difficult
to see at optical wavelengths, which is where past stud-
ies have focused. Scientists think such a glow, if it exists,
might be easier to detect at longer infrared wavelengths,
but no studies with the sensitivity needed to see this
faint scattered light have been performed yet.
However, dark matter does have mass and its gravity
can inf luence matter and light. So, dark matter does
contribute to a phenomenon called gravitational lensing,
in which a galaxy’s mass — including both its normal
and dark matter — causes the space-time around it to
curve. As light from an object in the background, such
as a more distant galaxy,
encou nters t h is c u r ved space-
time, it appears to bend, which
distorts and can even multiply
the image of the background
object. Astronomers do
observe this effect, and by
comparing the amount of
gravity necessary to do the
bending with the amount of
visible matter, they have used
it to confirm that galaxies are
enshrouded in massive halos
of dark matter.
Alison Klesman
Associate EditorGravitational lensing
occurs when a
foreground object,
such as this red galaxy,
distorts the space-time
around it, causing light
from a background
object — the distant
blue galaxy — to
appear bent as it
travels through the
curved space-time.
Astronomers can use
gravitational lensing to
confirm the presence
of dark matter, even
though it does not
appear to scatter or
otherwise interact
with light. ESA/HUBBLE & NASA