Be a considerate neighbor: Reduce nighttime glare.
Shield all your outside lights downward (or turn them off completely)
and enjoy the beautiful, starry night sky.
Night Sky—Winking stars
Like clockwork, Algol, in the constellation Perseus, winks for two
hours every 2.87 days. Algol was the first of its class, known as
eclipsing variable stars, to be discovered, and it’s still
the most famous one. Check on it daily, and you to will notice a
distinct change in brightness compared to its neighbors.
The name Algol means “demon star,” from “the
ghoul.” It probably received its name from its peculiar behavior.
In the constellation Perseus, it represents the eye of the Gorgon
The dimming wink is a result of one dimmer star passing in front
of another brighter companion. For this to happen, the stars must
be in orbit around each other. In Algol’s case, the orbit
takes 2.87 days. As the dimmer star, Algol B, passes in front of
the brighter star, Algol A, the amount of light reaching Earth temporarily
decreases. The normal magnitude is a fairly bright, constant 2.1,
but dims to 3.4 magnitude for two hours centered on the time of
Actually, Algol happens to be a triple-star system. The two stars
involved in the eclipsing are separated by only 5.8 million miles,
while the third star, Algol C, is at an average distance of two
hundred seventy-five million miles from the pair, and orbits them
every 1.86 years.
Currently, the Algol trio is 92.8 light years from Earth. But,
about 7.3 million years ago, it passed within 9.8 light years of
us. At that distance, it would have been much brighter than Sirius,
at approximately 2.5 magnitude. At that close distance, the gravity
may have distorted the Oort cloud of debris orbiting at the very
edge of our solar system. That distortion may have caused some of
the objects there to fall into the inner solar system as new comets.
The best time to see Algol this time of year is before sunrise.
It will be low in the northeast around 5:00 a.m. in May. Mid-month
will be best around the time of the New Moon. A good comparison
star is Almaak, magnitude 2.1, in Andromeda. You can compare Algol’s
brightness with them at a glance.
Algol is not the only eclipsing variable in the sky that can be
discerned by the naked eye. There are three others besides Algol
that we can see in the northern hemisphere.
Elthor, the lambda star in Taurus, is another eclipsing binary.
Its smaller range in magnitude, 3.4 to 3.9, and fourteen-hour eclipse
makes it more difficult to observe. This, too, is a triple-star
system. The two eclipsing stars orbit each other every 3.95 days.
The third, dimmer star has an orbital period of thirty-three days.
The Lambda Tauri system is approximately three hundred-seventy light
years from Earth.
Sheliak, the beta star in Lyrae, is another eclipsing binary,
but it appears different than Algol. The two stars of Beta Lyrae
are distorted by each other’s gravity and have become elongated
into an ellipse. As the stars orbit every 12.94 days, a continuous
change from bright to dim can be observed. The magnitude range is
between 3.3 and 4.4. There may be a third, very dim star in this
system as well.
Eta Geminorum, also called Tejat Prior, Propus or Praepes, is
a multiple-star system whose brighter component is a semi-regular
red giant. Most of the time, it varies very little from its normal
magnitude of 3.2. But every 8.4 years, the red giant is eclipsed
by its dimmer star, causing the pair’s total brightness to
drop to about 4.0. Again, there is a third companion star in this
group. This system is located three hundred-fifty light years from
So, find a nice, dark location and see if you, too, can see Algol,
Elthor, or Sheliak give you a wink.
THE PLANETS AND THE MOON
• Mercury will be paying a visit to the evening sky shortly
after sunset, low in the west, during the last half of the month.
On May 17, look for the Moon to join Mercury about forty-five minutes
after sunset in the northwest. The Moon will be a thin one-day-old
crescent. You may need binoculars to find Mercury.
• Venus will still be shining brightly in the west. It is
bright enough to find before dark.
• Mars is an early morning target, rising from 4:00 a.m. early
in the month, to 3:00 a.m. late in the month. On Mother’s
Day, May 13, you can find the Moon and Mars above the eastern horizon
an hour before sunrise.
• Look for Jupiter to rise from about 11:00 p.m. to 8:30 p.m.
as the month progresses. One hour after sunset on May 31, you have
the chance to see the Full Moon and Jupiter together. Antares is
also in the mix, just above the moon.
• Saturn is forty-five degrees above the western horizon around
9:30 p.m. Saturn and the Moon join up on May 22, about two hours
after sunset. Saturn will be to the lower right of the Moon, and
Regulus will be to the upper left of the Moon. Procyon will be on
the horizon almost due west.
• The Moon is full at 4:09 a.m. MDT on May 2. There will be
a “Blue” Moon, the second Full Moon in the same month,
on May 31. The New Moon is on May 16 at 1:27 p.m. MDT. Lunar perigee
at 223,315 miles from Earth is on May 15, and apogee at 251,939
miles is on May 27.
If you have a question or comment for Charlie,
you may email him at email@example.com.