Halloween full moon a real treat
Although all the little ghosts, goblins, witches, and zombies out on Halloween night will be focused on collecting as many treats as possible, there is one treat which, if the weather cooperates, will add to the overall spookiness of All Hallows Eve: a rare full blue moon.
The full moon on Oct. 31 is what is referred to as a blue moon, the second full moon within a calendar month. The first occurred on Oct. 1 (the harvest moon).
The term blue moon has nothing to do with the actual colour of the moon, although there are occasions when, if there is a large volume of particulate matter in the air (such as after a major volcanic eruption), the moon may have a slightly bluish tinge to it when viewed from areas near the eruption. If the particulate material (ash and fine dust) is of a certain size (one micron in width) it will scatter the red wavelengths of the moon's light, allowing other colours to pass through the atmosphere and reach Earth's surface.
If you recall my column regarding why the sky is blue, you will remember the human eye is more sensitive to the blue end of the light spectrum, and, thus, sees the moon's colour on such occasions as a bluish tinge (a tinge rather than a full blue colour because the moon's light is reflected sunlight, and thus fainter).
The term blue moon was initially used (and still is today) by astronomers to refer to the phenomenon of four full moons in any given season, as opposed to the standard three (one per month). In 1946, amateur astronomer, James H. Pruett, wrote an article for an astronomy magazine in which he incorrectly associated the term with the second full moon occurring in a given month. Subsequent media use has entrenched this misinterpretation into modern use.
TWO FULL MOONS IN ONE MONTH?
Our moon has an approximate 29.53-day cycle (referred to as a synodic month or synodic
period), the period between the same lunar phases (full moon to full moon). As this is close to the length of our calendar months, most months have one full moon. However, our calendar is based on the Earth's orbital period (365.256 days; called a sidereal year), not lunar phases. This means over time, the synodic period doesn't exactly line up with the lengths of the months. As a result, on average, every 2.7 years, two full moons occur in one month (as in this month).
An Oct. 31 full moon occurs roughly once every 19 years. This is referred to as the Metonic cycle, discovered by Greek astronomer Meton in 432 BC. However, this cycle is only accurate to within one day, due to variations in the moon's orbit around the Earth, and the number of leap days in the 19-year period.
That means the full moon can fall on Nov. 1 (as it did in 1963, 1982, and 2001), or on Oct. 31 (as in 1925 and 1944).
The next Halloween full moon won't occur until Oct. 31, 2039, making this month's full Halloween blue moon a rare treat.
When next you hear the
expression "once in a blue moon," you will understand that the speaker is probably referring to events that happen infrequently.
October's full moon is also known as the hunter's moon, referring to the now-outdated practice of hunting by the light of the full moon during this time of the year. Because the moon will be at apogee (its farthest distance from the Earth) on Oct. 30, the slightly, post-apogee full moon on Oct. 31 could be referred to, in some media articles, as a micro-moon (as opposed to a supermoon, when the full moon is at perigee, or closest to Earth).
THIS WEEK’S SKY
Mercury, having passed inferior solar conjunction on Oct. 25, is still too close to the sun to be observed.
Venus (magnitude -4.02) is still visible in the eastern, pre-dawn sky. It rises around 4:35 a.m., reaching an altitude of 27 degrees above the southeast horizon, before fading from view as dawn breaks around 7:30 a.m. Venus reaches perihelion (closest orbital distance to the sun) on Oct. 30.
Mars (magnitude -2.3), just
past opposition with the sun on Oct. 13, shines brightly eight degrees above the eastern evening horizon around 6:25 p.m., reaching a height of 48 degrees above the southern horizon before disappearing from sight when it drops below eight degrees above the western horizon at about 5:30 a.m. Look for the red planet to the upper right of the full Halloween blue moon on All Hallow's Eve.
Jupiter and Saturn remain early evening objects. Jupiter (magnitude -2.2) is visible around 6:25 p.m., 21 degrees above the southern horizon, then sinks towards the horizon and sets around 10:40 p.m. Saturn (magnitude +0.57) follows Jupiter into the southern sky around 6:45 p.m., 22 degrees above the horizon, remaining visible until it sinks below 10 degrees above the southwest horizon around 9:45 p.m.
ZODIACAL LIGHT COMING UP
Weather permitting, the next couple of weeks will be an optimum time to view the zodiacal light.
Visible as a diffuse, roughly pyramid-shaped cone of light above the eastern, pre-dawn horizon (about where the sun rises), this celestial phenomenon is best seen under a clear, moonless sky away from city lights. It is caused by sunlight reflecting of myriad dust particles within the inner solar system, and is most prominent when the ecliptic (the apparent path of the sun, moon, and planets across the sky) is tilted upwards relative to the horizon, as it is in both the autumn (above the eastern horizon before sunrise) and spring (above the western horizon after sunset).
Until next week, clear skies, and happy Halloween.
EVENTS
Oct. 30 — Moon at apogee (farthest orbital distance from Earth) and Venus at perihelion (closest orbital distance to the sun)
Oct. 31 — Full moon (also known as the blue moon or hunter's moon)
Nov. 1 — Mercury at aphelion (farthest orbital distance from the sun)
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. His column, Atlantic Skies, appears every two weeks.