THE SOUTHWORTH PLANETARIUM
207-780-4249       www.usm.maine.edu/planet
70 Falmouth Street  Portland, Maine 04103
43.6667° N,                    70.2667° W
Founded January 1970
            "aut viam inveniam aut faciam"
                     "I will find a way or make one."
                                 -Spoken by Hannibal, ancient Rome's
most formidable adversary
                                       -Spoken also much later by
Robert Peary, the Maine native who, with Matthew Henson, discovered
the North Pole





THE DAILY ASTRONOMER
Wednesday, March 2, 2016
Lasers to the Moon


Today, we have enough time to pull out only one Pandora parchment.
Pandora parchments are slips of paper onto which subscriber questions
have been inscribed.    We collect them and, when time permits, answer
them in turn.   We sincerely wish we had more time to contend with
these queries, as we love to answer questions.   (Or, to be fair, we
love to attempt to answer questions.)        Today's Pandora parchment
pertains to partially to an issue that has lately been in the news.
While we're only answering one parchment, it contains a few questions.

"I heard on the news that people have been shining hand held lasers
into airplane cockpits.      How far do they go?  Can the light from a
hand held laser reach the moon?     I know that astronomers use them
at star parties.   Can you see the beam on the sky?"
                 -S.L, Raymond


 Recently, many commercial jet pilots have reported having seen lasers
shining into their cockpits.   A few times, a pilot has actually had
the light shine in his/her eyes, prompting an immediate diversion to
the nearest airport.  All of the lasers used in these EXTREMELY
DANGEROUS PRANKS are hand-held devices that astronomers often use when
presiding over star parties.    These incidents have been so frequent
(-about 9000 reported sightings) that the FAA has lobbied Congress to
enact legislation to criminalize possession of these labors.*

The beams can shine in cockpits because some of the most powerful
lasers have 5 - 10 mile ranges, high enough to reach planes even at
cruising altitudes.    Such a beam cannot, however, reach the moon,
which, even when at its least distance (perigee), is about 238,000
miles away.  Even powerful laser beams will disperse and be absorbed
by the intervening atmosphere before they can even extend out of the
atmosphere, let alone each the moon.

While a laser dot will not actually appear on the sky like the red
dots of planetarium lasers reflect off the dome, the beam will often
be distinctive enough to guide observes toward any given celestial
object.   In a humid sky, one can often perceive a beam like a thin,
glittering rod stretching up into the sky.    On a foggy night, the
beam is so distinct as to almost seem like a light saber, albeit a
pathetically thin one.         Perhaps the beam would show as a dot on
low hanging and thick clouds, but even this is unlikely as clouds are
merely fog banks suspended in air.

These lasers are very handy at star parties because they can help
guide observers throughout the sky,  Of course, it wouldn't surprise
us if they were no longer legal in a couple years.


*As this is literally a life and death issue when dealing with lasers
in cockpits, we can well understand the FAA position.