The solar eclipse on Thursday September 1st, will have been brought to us by the nodes. No the nodes are not a scientific nor a musical group, but rather the nodes represent an intersection between the orbital path of our Moon, or another planet, with the Earth’s orbital path, the plane of the ecliptic.
There are two nodes or intersections, the ascending node and the descending node. There are two nodes because the planets as well as our Moon do not orbit the Sun on the same level, or plane as does the Earth. Their respective orbits are inclined (tilted) away from the plane of the ecliptic by varying amounts such that they will at times appear below or above the plane of the ecliptic. There will be two times each orbit around the Sun where the planet or our Moon will be on the plane of the ecliptic as it crosses moving from below setting up the ascending node or from above toward below, setting up the descending node.
So what is the significance of the nodes? The significance is all about timing. If the time of the new Moon phase, for example, occurs at or near the time for a node crossing then there will be a solar eclipse. Remember that at this moment the Moon is on the plane of the ecliptic and is more or less directly between the Earth and the Sun. More importantly, if the times are exact or very close there will be either a total solar eclipse or an annular solar eclipse. During an annular eclipse the Moon is somewhere near its apogee, or most distant point for that particular orbit. This annular eclipse happens 5 days before the Moon reaches apogee on September 6th but the Moon is still far enough away so that its apparent diameter is less than the Sun’s apparent diameter. Both are around 0.5o or 30′ in apparent diameter, however for this annular eclipse the Moon will be about 3% smaller and not be able to completely cover the Sun at mid-eclipse. Instead at mid-eclipse there will be a ‘ring of fire’, the annulus, around the Moon.
Thursday September 1st the new Moon phase is at 9:03 UT and the Moon is at its ascending node about 6 hours later at 15:27 UT. The Moon makes first contact with the Sun at 6:23 UT; maximum or mid-eclipse is at 9:06 UT; and the eclipse officially ends with last contact at 12:00 UT.
To sort of complete this story, if there is a solar eclipse, no matter how total or less than total, there will be a lunar eclipse two weeks away at full Moon phase. Eclipses occur in pairs so this pair will be completed with the penumbral lunar eclipse at full Moon on September 16th.Full Moon on September 16th is at 19:05 UT, about 19 hours after the Moon is at its descending node, 11:57 UT September 15th. Because of the time difference, the angle the Moon follows through the Earth’s shadows does not cross the dark inner umbral shadow but rather only takes it across the less noticeable outer penumbral shadow. Unless you knew about it you may not notice a slight dimming of the reflected moonlight.
For an outstanding web site about all eclipses including information for these two eclipses go to the Hermit Eclipse web site.
Click here to go to the Qué tal in the Current Skies web site for monthly observing information, or here to go to bobs-spaces.