Ascending Node
Published on May 4, 2007 at 4:18 pm.
4 Comments.
Filed under astronomy.
Looking for something to blog about, I looked in my Observer’s Handbook under today’s date. It said “Mercury at Ascending Node.” OK, cool. I’ll write about that. So, what is this “ascending node” stuff all about?
The terms ascending node and descending node come from a study of planetary orbits. To start with, consider Earth’s orbit around the Sun. Imagine that the Sun is not really as bright as it is, but rather is as dim as the Moon. Then, you’d still be able to see the stars when the Sun is up in the sky. As Earth orbits the Sun, from our perspective on Earth, the Sun would appear to be in front of different constellations as our perspective on it changed with Earth’s changing position in its orbit. The Sun would appear to sweep out a path in the heavens that we call the ecliptic.
Now, all of the major planets orbit the Sun with orbits that are nearly in the same plane. But, nearly in the same plane does not mean exactly in the same plane. Rather the orbits are tilted a few degrees with respect to one another. Remember that the ecliptic is swept out by the Earth’s motion around the Sun, so if another body’s orbit is tilted with respect to the Earth’s orbit, then it will appear tilted with respect to the ecliptic. However, all of the orbits do go around the Sun, so some portion of the orbit will be north of the ecliptic and some portion will be south of the ecliptic. The figure above shows this arrangement, as viewed from the perspective of an observer outside the Solar System and stationary with respect to the Sun. For an observer on Earth, the planet moving along its orbit would also move from one side of the ecliptic to the other side. Again we define this as north and south of the ecliptic. But, obviously, as the planet moves from north of the ecliptic to south of the ecliptic, or from south to north, then it must cross the ecliptic. The points in the sky where the orbits cross are called nodes. A line drawn from one node to another is called the line of nodes.
If a planet is at its ascending node, than that means that it is crossing the ecliptic moving from south to north. When the planet is at its descending node, then it is crossing the ecliptic moving from north to south. So, to prepare for my bog posting, I used Starry Night Pro planetarium software to show Mercury and it’s orbit. (Click on the picture for a larger view.)
Here you see the Sun and Mercury. I also have the ecliptic marked as well as Mercury’s orbit. Hmm. This was made for about 2:15 pm CDT here in Texas. The node is the point where Mercury crosses the ecliptic. Mercury is currently on the far side of its orbit and is moving away from the Sun, as seen from our vantage point of Earth. Oh, but wait! Didn’t I start this post saying that Mercury was at its ascending node? But, if you look at the diagram, clearly Mercury isn’t at the node. What gives? And, what is that other line there above the Sun and Mercury? To answer the second question, lets pull back to a lower magnification. I can also turn off the daylight, and so I did that to give the next picture.
Here, you can see all of Mercury’s orbit. This makes it clear what that second line is. It is also Mercury’s orbit! Remember, Mercury’s orbit is tilted with respect to Earth’s orbit. And, Earth is not oriented at the moment in a position along the line of nodes. So, we are seeing the orbit from an angle. So, we get this skewed perspective. Now, if Earth were oriented along the line of nodes, then the orbit of Mercury would pass directly in front of the face of the Sun. If Mercury happened to be at that point in its orbit at the same time, then we’d be able to see it transit in front of the Sun, as it did this past November, when I took these photographs.
So, that is all fine, but it doesn’t answer the question as to why Mercury doesn’t appear at the node. That is simple. It is too late. Mercury had already passed that point by the time that I had the software set to show. Mercury was at the ascending node on Friday, May 4, 2007, yes, but it was at about 04:18 Universal Time. That actually was at 11:18 pm Central Daylight Time on Thursday, May 3, here in Texas. Yes, Mercury moved that much in those fifteen hours. So, I reset the time to when Mercury was actually at the ascending node. Alas, the Sun and Mercury were well below the horizon then, and not visible from Texas. But, the nice thing about software is that you can very easily move to some other place. So, I set the location for some place in China, and got this view:
Now, you can see Mercury at the ascending node.
And, now you know that the term ascending node means. Mercury will continue to orbit the Sun, of course, and eventually it will be back onto the part of its orbit that will appear south of the ecliptic as seen from Earth. When that happens it will have to cross the ecliptic again, and that will be the time that it will be at its descending node. That will be on June 11, 2007.
Mercury isn’t the only planet to have ascending and descending nodes, of course. Since all of the planetary orbits are tilted a bit from Earth’s then will all appear at various times to be either north or south of the ecliptic. But, Mercury, being the closest to the Sun, is the fastest moving planet, so it moves back and forth along its orbit faster than the others, and thus it moves from ascending to descending nodes and back to ascending node faster than the other planets.
-Astroprof
Orbit diagram courtesy of Wikimedia
Sky views created using Starry Night Pro
Ed Davies on May 5, 2007 at 5:37 am: 1
It’s worth adding that for Earth orbiting satellites the terms ascending and descending node are used for the points where the satellite cross the equator, northbound and southbound respectively.
A Ler…-- Rastos de Luz on May 7, 2007 at 4:25 pm: 2
[…] “Ascending Node“, no Astroprof’s Page; […]
Anantha Krishna on January 27, 2009 at 5:16 am: 3
I think these ascending and descending nodes are not stationary. They are also changing their positions as a particular panets revolves round the sun. please clarify.
Astroprof on January 27, 2009 at 7:22 am: 4
They do move a bit. That is called precession. The ascending node that I was talking about here as the apparent ascending node, where the orbit appears to cross the ecliptic. Since the orbits of both Earth and Mercury precess, the relative orientation of the planets change, and thus the nodes appear to move a bit.