28 September 2015, “super” Moon total eclipse: an observer guide
Next Sept. 28, the wonderful show of a total lunar eclipse will be back in our skies. A great opportunity for many of us to look up and enjoy this cosmic beauty.
The observation of a total lunar eclipse can easily be done by naked eye: the plainnes of this kind of vision makes part of the event’s spectacularity. Of course, a binocular or a telescope will offer a much larger image, but those with no optical tools will not miss anything essential.
What is the total lunar eclipse.
It is well known that, in less than one month, the Moon describes an elliptic orbit around the Earth, while the latter moves around the Sun, which lights up both. For a lunar eclipse to happen, the Moon must enter into the Earth’s shadow, so receiving no direct light from the Sun: it looks evident that the alignment needed for this is the Sun-Earth-Moon one, with the first and last objects visible from the Earth on opposite directions. That is, after all, the geometrical condition for the full Moon, hence the obvious, mandatory full Moon needed for a lunar eclipse to happen.
Clearly, the full Moon is not enough for an eclipse to happen, otherwise we would have one on every lunation (the same would happen with solar eclipses): it is needed for the mentioned Sun-Earth-Moon alignment to be good enough, withing very narrow margins, making the difference among a total eclipse, a partial one and… no eclipse at all.
The lunar orbit plane does not coincide with the one of the Earth’s orbit (which projection in the sky defines the line of the ecliptic, crossing the Zodiacal constellations and on which our Sun is apparently moving), having a relative inclination of about 5 degrees. Looking at the motion of the Moon in the sky, we will see it above or below the ecliptic and there will be two points, called nodes, where it crosses the ecliptic.
If the full phase is reached when the Moon is close enough to one of the nodes, then the mentioned alignment with the Sun is warranted (it helps reminding that the Sun is, by definition, on the ecliptic, the latter being the projection on the sky of the Earth’s orbit around the same star, therefore when the full Moon is in one of the nodes, the antithetic alignment is satisfied). And this is what will happen next 28 Sept.
The phenomenon of the eclipse develops gradually. The Moon stars entering into the Earth’s penumbra cone, showing a marginally darker border on the east side of its disk. Later, the satellite jumps into the umbra, which will produce a much stronger darkening, initially limited to a small part, but soon extended to a larger part of the lunar disk. When the Moon will be entirely inside the umbra cone, the totality will start. It will reach its maximum, the turning point of the event, then the eclipse will develop symmetrically, leaving the umbra and later the penumbra, with the Moon coming back to the full and direct solar light.
During the totality, but also during the advanced partiality, the Moon shows a characteristic, elegant bronze color. In those moments, the terrestrial atmosphere is addressing to the Moon the red part of the solar light (it is worth to note that what is for us a lunar eclipse, from the Moon is a solar one: during the totality, from the Moon we would see a red halo all around the Earth, that is our atmosphere, sending to the Moon the red part of the spectrum).
At the totality, eclipse can look more or less dark: this depends on the trasparency of our atmosphere, which is affected, for example, by strong volcanic eruptions.
If the alignment of the three involved bodies is not perfect, the eclipse will be only partial, possibly of mere penumbra.
The 28 Sept. 2015 full Moon.
To crown it all, the Moon involved in the upcoming eclipse is not a ordinary full one. On 01:47 UT this 28 Sept., our satellite will be at its minimum distance from our home planet, that is at its perigee, namely at 356876 km from us (versus a mean distance of 384400 km; the eccentricity of the lunar orbit is the source of this difference). One hour and five minutes later, the Moon will be full. In short, that is a so-called full “supermoon“, but a special one: it is the closest one of 2015, this making its angular diameter the largest of the year.
The 28 Sept. eclipse.
The upcoming, 28 Sept. eclipse will be visible from most Europe, most of the Americas and part of Africa. It is the second lunar eclipse of 2015 (after the one last April) and it is the last one of a series of four started on April 2014 (all the four lunar eclipses of 2015 and 2014 were total)
The circumstances of the event are well-summarized in the following Nasa graphic:
Times are given in Universal Time, be sure to correct them for your very own timezone.
Being the eclipse so spectacular, you can try to capture stunning pictures showing the red Moon and something special in your urban/natural environment. In Europe, in particular, the totality will peak with the Moon not far from the western horizon, early in the morning. Such a low Moon can help finding a good combination of terrestrial elements to be included in your pictures.
Keep in mind that the Moon, at totality, is a MUCH fainter than usual: if 1/125 s (at 100 ISO and f/11) can be enough for the no-eclipsed full Moon (like a normal terrestrial landscape, after all), totality can require a minute or so of exposure, making life harder for the apparent est-to-west motion of the sky, due to the Earth rotation. If you do not have a proper device tracking that motion, you will have to find the best combination of ISO/noise and sky trailing.
Also, the lunar disk will have a dimension in millimeter like 1/100 of the focal length of the optics you will use.
The Team of the Virtual Telescope Project wishes a great eclipse to all of you!
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