Sky & Telescope - October 2020 - 38

Artificial Satellites

the satellite's reﬂective solar panels
direct sunlight earthward. On such
occasions, for a lucky observer in the
right location, a geosat can brighten
by orders of magnitude, ﬂaring to
naked-eye visibility.

W GONE IN A FLASH For two decades,
catching an Iridium satellite flare was a favorite sport among skywatchers. However,
the last of the 66 original Iridium satellites
was deorbited in December 2019, bringing
the era to a close. In this photo, a satellite flares to brightness then fades away in
twilight, southeast of Altair in Aquila.

A Flash in the Night
of displacement depends on your
latitude and is given in the table
found on page 40.
As the table also shows, geosat ﬂares are visible only around
the equinoxes. Each viewing cycle
is divided into one phase for the
Northern Hemisphere and one for
the Southern Hemisphere, with
a brief interval between in which
Earth eclipses the Sun and no ﬂares
are seen. In Northern Hemisphere
autumn, the ﬁrst ﬂares appear after the equinox, and as the
ﬂare season progresses they move eastward away from near
the First Point of Aries, which is actually situated southeast of
the Circlet of Pisces. In Northern Hemisphere spring, ﬂares
happen before the vernal equinox in March and appear east
of the First Point of Libra (where the ecliptic and the celestial
equator meet), southeast of Beta (β) Virginis. The position of
the ﬂares migrates westward toward that point until the end
of the ﬂare season.

Playing the Angles
Over any location, the Clarke Belt geosats form an invisible
band of solar panels stretching across the sky near the celestial equator from horizon to horizon. All the geosat solar panels remain aligned in the same general orientation facing the
Sun. This means that, if we could see them, the geosats in the
arc above would appear from our earthbound perspective to be
pivoting in unison, completing one 360° revolution per day.
When the largest geosats ﬂare, they rise to naked-eye visibility and rival the brightest stars. You can follow their fading for almost a quarter hour. And because they're stationary,
you can observe geosats in a non-tracking telescope without
having to constantly adjust its aim.
Geosat ﬂares become visible starting with those low in the
east at dusk. Over the course of the evening, as the antisolar
point (the spot in the sky directly opposite the Sun's position)
rises, geosats appear in succession, one after another, higher
and higher in the sky.
A night's display of geosat ﬂares progresses with ﬂares in
the east at dusk, to dawn with ﬂares appearing near the western horizon. Sometimes, there are no visible ﬂares for a while;
W EARTH'S RING Conspicuous in this computer-generated plot showing
objects in Earth orbit is the Clarke Belt of geostationary satellites, located
at a distance of 35,785 km (22,236 miles) above the equator.

OCTOBER 2 02 0 * SK Y & TELESCOPE

IRIDIU M FL A RE: G A RY SERONIK; SIM UL ATION: N ASA ODPO

Most readers have witnessed the
startlingly bright appearance of an
Iridium satellite. At times, these
objects exceeded the brilliance of
Venus. But now that the last Iridium
has been deorbited, observing geosat
ﬂares is gaining popularity.
Geosat ﬂares are quite different
from either Iridium ﬂares or the
sequence of ﬂashes produced by tumbling satellites. Instead of drifting
across the starry background, geosat
glints appear as ﬁ xed as actual stars, barely moving before
fading away over a span of 10 to 15 minutes. In some cases,
they brighten, disappear, then rebrighten before winking out
for good. If you know when and where to look, you can see
dozens of geosat ﬂares over the course of a single evening.
Although geosats are faintly visible in telescopes all night
long on every night of the year, their ﬂares depend on a
speciﬁc geometric alignment between the satellite, the Sun,
and the Earth. This happens only a few nights per year. And
as with Iridium ﬂares, the visibility of geosat ﬂares strongly
depends on the observer's speciﬁc location.
Since the geosats are in equatorial orbit, they are seen
from Earth along the great circle of the celestial equator (CE).
However, their positions are displaced by parallax south of
the CE for observers in the Northern Hemisphere, and north
of the CE for those in the Southern Hemisphere. The amount

Sky & Telescope - October 2020

Table of Contents for the Digital Edition of Sky & Telescope - October 2020