Spring brings with it a penumbral lunar eclipse
The vernal equinox occurs here in the Atlantic region at 12:09 a.m. ADT (12:39 a.m. NDT) on Wednesday, March 20. This marks the official astronomical beginning of the spring season here in the northern hemisphere, and the beginning of the autumn season in the southern hemisphere.
The term "equinox" is from the Latin aequinoctium, derived from aequus (meaning "equal") and nox (meaning "night"), essentially equal lengths of day and night (more on that later).
Astronomically, the start of the spring season begins the moment the sun reaches the point in the sky directly over Earth's equator (referred to as the "subpolar point"), as it moves from the southern hemisphere northward into the northern hemisphere. Another way of understanding it is that, at the spring equinox, the sun crosses the celestial equator (an imaginary line extending outward into space from Earth's equator) moving northward into the northern half of the imaginary celestial sphere surrounding the Earth.
A similar event occurs when, in September, the sun crosses the celestial equator heading south into the southern celestial hemisphere, marking the commencement of the autumn season here in the northern hemisphere. At the time of an equinox, the Earth's rotational axis is directly perpendicular to the Earth-sun line, tilting neither toward nor away from the sun.
Equinoxes are also the only times when the solar terminator (the moving line dividing the day and night sides of the Earth as seen from space) is perpendicular to Earth's equator.
MULTIPLE EQUINOXES
Of course, it is not actually the sun that is moving, but the position of the Earth relative to the sun that has changed at this moment in time as it orbits the sun. There are two equinoxes annually — the vernal/spring equinox in March (thus its other name, the March equinox) and the autumnal/fall equinox in September.
Both hemispheres experience the equinoxes in the same month, only the seasons are reversed (i.e., spring in the north, autumn in the south; summer in the north, winter in the north, and so on).
At the equinoxes, both hemispheres received approximately the same amount of sunlight. It should be remembered, though, that the Earth doesn't orbit the sun in an upright manner, as its rotational axis is always tilted by approximately 23.5 degrees relative to the sun.
BY DEFINITION
While the English translation of the Latin term aequinoctium would have one believe that the length of the day and the length of the night periods are equal, this is not exactly correct; there is slightly more daylight than night on the day of an equinox. The difference in the length of day and night at an equinox is due to a number of factors — the angular size (how large a sphere or circle appears from any given location) of the sun, atmospheric refraction (the deviation or bending of light from a straight line as a result of different atmospheric densities) and differences in latitude.
Equinoxes are the only two times of the year that the sun rises due east and sets due west for everyone on Earth. Although we refer to and celebrate the date of the equinox as if the event lasted the entire day, it is actually an exact moment in time, i.e. the exact moment the central point of the sun crosses the celestial equator.
TIME ZONES
The equinoxes (as well as the solstices) are based on Coordinated Universal Time (UTC) — Greenwich Mean Time (GMT) in the United Kingdom and, while the equinoxes (and solstices) occur at the same moment in time across the globe, the actual date and local time of the event varies from place to place depending on the year and a location's time zone.
The civic calendar date for the first day of spring (as well as the dates of the other three seasons) shifts from year to year, and can occur on either March 19, 20 or 21.
When spring arrives in any given year is related to the establishment of the Gregorian calendar (the calendar most of the modern world now uses) by Pope Gregory XIII in 1582 to modify and replace the Julian calendar (established by Julius Caesar in 46 BC). In a non-leap year (365 days), the moment of the vernal equinox is approximately five hours, 49 minutes (UTC) after the time of the equinox in the previous year, while in a leap year (366 days), it occurs approximately 18 hours, 11 minutes (UTC) before the time in the previous year. Balancing the increases of the non-leap years against the losses of the leap year keeps the date of the vernal equinox from drifting more than one day from March 20 each year.
WHEN DOES A LUNAR ECLIPSE OCCUR?
A lunar eclipse can only occur at the time of a full moon, which occurs for us here in the Atlantic region at 4 a.m. ADT (4:30 a.m. NDT) on the morning of March 25. There are three types of lunar eclipses: total, partial and penumbral.
• A total lunar eclipse is when the moon passes totally inside the Earth's umbral shadow (the darker, inner circle of the two-zone, "bull'seye" shadow that Earth casts into space as a result of the sun's light on it);
• A partial lunar eclipse is when the Moon passes partially through the penumbral shadow (the outer, fainter shadow of the bull's-eye pattern) and partially through a portion of the umbral shadow (the familiar "bite-out-of-themoon" lunar eclipse event);
• A penumbral lunar eclipse is when the Moon passes through only the penumbral shadow, and does not intrude into the umbral shadow.
After midnight March 25, the moon will pass through the Earth's penumbral shadow. The penumbral lunar eclipse will begin at 1:52 a.m. ADT (2:22 a.m. NDT) and end at 6:33 a.m. ADT (7:03 NDT), with the "greatest eclipse" (when nearly all of the moon will be within the penumbral shadow) occurring at 4:12 a.m. ADT (4:42 a.m. NDT), with the entire event lasting just over 4.6 hours. Although the entire moon won't pass totally within the penumbral shadow at "greatest eclipse" (a tiny portion of it will still be visible at the top), it is referred to as a "deep penumbral lunar eclipse." While there won't be any visible "bite" out of the moon, the moon's surface will turn a subtle shade of grey, which, depending on the weather, seeing conditions, and your eyesight, may not be very apparent.
THIS WEEK'S SKY
Mercury (mag. -1.3, in Pisces – the Fish), having recently completed superior solar conjunction, is briefly visible around 7:40 p.m., 10 degrees above the western horizon as dusk yields to darkness, before sinking toward the horizon and setting around 8:45 p.m.
On the 24th, Mercury will reach its greatest elongation (visual angular separation) east of the Sun.
Venus (-3.9, in Aquarius – the Water-bearer), only 1 degree above the southeast horizon at dawn, is not observable this week.
Likewise, Mars (mag. +1.2, in Capricornus – the Sea Goat) is only 1 degree above the southeast horizon by dawn, and not observable.
Jupiter (mag. -2.2, in Aries – the Ram), heading toward superior solar conjunction, becomes visible at approximately 7:40 p.m., 36 degrees above the western horizon, before sinking toward the horizon and setting by about 11:25 p.m.
Saturn (mag. +1.0, in Aquarius) is not observable this week, as it sits just 3 degrees above the southeast horizon at dawn.
Uranus (mag. +5.8, in Aries) becomes visible around 8:30 p.m., 33 degrees above the western horizon, where it will appear just to the upper left of Jupiter, before it sinks towards the horizon and sets shortly before midnight.
Neptune (mag. +8.0, in Pisces), having recently passed behind the Sun, is only 1 degree separation from it, and, therefore, not observable.
'WORM MOON'
The Full Moon on March 25 is sometimes referred to as the "Worm Moon," as the month of March is often the time when earthworms begin to emerge from the thawed spring soil, an event that does not go unnoticed by many bait anglers.
Until next week, Happy Spring, and clear skies.