Eclipse and the Nodes

Click on graphic to see it full size.

Click on graphic to see it full size.

   The solar eclipse on Sunday 3 November, 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.

Eclipse animation

Eclipse Animation

   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. On 3 November the new Moon phase is 12:48 UT 6:48 am CST) and is at its ascending node nearly 6 hours earlier at 6:55 UT (12:55 am CST). The Moon makes first contact with the Sun at 10:04 UT (4:04 am CST); maximum eclipse, mid-eclipse, is at 12:46 UT (6:46 am CST); and the eclipse officially ends with last contact at 14:27 UT (8:47 am CST).
    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 started with the penumbral lunar eclipse last month at full Moon on 18 October.
Moon Grazing the Earth's Shadow

Moon Grazing the Earth’s Shadow

Since that occurred about 26 hours before the Moon was at its descending node the angle the Moon followed through the Earth’s shadows only had it ‘graze’ the less noticeable outer penumbral shadow. Unless you knew about it you would have not noticed a slight dimming of the reflected moonlight. And, because the Moon’s orbital path is inclined, this lunar eclipse which was at descending node means that the 3 November solar eclipse will be at its ascending node – which it will be.

   Mercury is also in the picture because it too is at its ascending node and just a few days ago Mercury was at inferior conjunction. Had these two, ascending node and inferior conjunction, coincided we would have had a transit of the Sun by Mercury.
Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.

Dance of the Planets

23 May to 11 June - 8:30 pm CDT

23 May to 11 June – 8:30 pm CDT

   In my posting yesterday I described the orbital motions of these three planets without the use of a graphic to help visualize that explanation. This animated graphic that is set to one-day intervals starts with today’s date and then runs for a few weeks. This gives enough time to see how both Mercury and Venus are moving eastward while it appears that Jupiter is moving in the opposite direction, west towards the Sun. Jupiter is in fact moving eastward but considerably slower than the Sun’s apparent eastward motion. The net effect is that Jupiter sets earlier and earlier as the Sun gets closer, and eventually Jupiter moves behind the Sun.

   Both Mercury and Venus were recently on the opposite side of the Sun from the Earth at superior conjunction. Coming from that relative position both planets travel eastward until they reach a point with respect to the Sun that we see as the furthest apart, or the greatest angular separation between the Sun and Mercury or Venus. This happens on either side of the Sun and are known as elongations. Since both planets are on the east side of the Sun they are both heading toward eastern elongation. In the animated graphic above you can see all of Mercury’s orbit on the east side of the Sun including the part where Mercury reaches eastern elongation and heads back toward the Sun. Because Venus is further from the sun it has a longer orbital period around the sun and likewise a longer orbital path so Venus’s orbital path on the graphic extends past the edge.


   Click here to go to the Qué tal in the Current Skies web site for more observing information.