On March and April evenings, planets that are in the western sky, also the young Moon, are liable to come into view more easily. Why?
The reason is the angle of the ecliptic – the path along which these bodies travel, as does the Sun itself. At this time of year, the ecliptic slopes up more steeply from the evening horizon. Therefore any body not far to the east (left) of the Sun will be relatively higher as the Sun sets, will be in darker sky, and will take longer to follow the Sun down to the horizon.
Here is the evening scene for the date of the spring equinox, March 22.
See the end note about enlarging these illustrations.
We show the Sun below the horizon, so you can see that at this date it reaches the “ascending node,” the place where the ecliptic crosses northward through the celestial equator. It is the middle of the part of the ecliptic that slopes most steeply northward.
The broad arrow on the celestial equator shows how fast the sky appears to rotate. It shows how far a star or planet is carried down toward the horizon in one hour.
The scene on this date in this year is rather lacking in planets to illustrate the point. It also lacks the Moon, because the Moon is Full on March 21. So here’s the same scene earlier in the month, when the Moon was making its rapid appearance.
The figures to the right of the Moon’s positions are its “age.” At this time on the evening of March 7 it was only 33 hours “old,” that is, past its New position beside the Sun, and was down at the horizon and vanishingly thin. The next evening it was 57 hours old, and on the actual date of the scene 87, and you could probably see it.
The slope of the ecliptic depends on your latitude – how far north or south you are on the spherical Earth. The preceding pictures were for a typical U.S.A. latitude of 40° north. Here’s what it looks like if you’re 12° farther north, in the middle of England.
The ecliptic’s slope is lower. If you were to go sledding on to the North Pole, the celestial equator would of course be flat along the horizon, and the ecliptic would rise up mildly from it, only at its “inclination” of about 23°.
Conversely, if you go south, the angle of the ecliptic will steepen. When you arrive at latitude 23° north – the Tropic of Cancer – the ecliptic will be vertical.
On southward, the equator and ecliptic seem to topple over! They slope up in the evening not leftward but rightward. You’d have to get used to that if you migrate to New Zealand.
For the southern hemisphere, this is not the spring but the autumn equinox.
By comparison, here is the evening scene at the September equinox, autumn for the northern hemisphere, with the ecliptic at a low angle –
– and spring for the southern, with steep ecliptic:
The angle of the ecliptic to the horizon at certain times of year has a special, and paradoxical, effect on the observability of one planet: Mercury. But that’s a whole other topic.
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DIAGRAMS in these posts are made with precision but have to be inserted in another format. You may be able to enlarge them on your monitor. One way: right-click, and choose “View image”, then enlarge. Or choose “Copy image”, then put it on your desktop, then open it. On an iPad or phone, use the finger gesture that enlarges (spreading with two fingers, or tapping and dragging with three fingers).
I’ve always seen ascending and descending node refer to the points where the Moon or a planet crosses the ecliptic. This is the first time I’ve seen the first point of Aries, the point where the Sun crosses the celestial equator, referred to as the Sun’s ascending node. I looked up “node” in the Merriam Webster dictionary, and there is a meaning for the point where an artificial satellite crosses the equator, but no reference to the Sun. Is this another meaning of node?
Node (Latin nodus, “knot”) is used in various sciences for points interrupting lineare things (e.g. lymph nodes), and in geometry and astronomy for the intersections of planes. There’s quite a long paragraph on it in my “Albedo to Zodiac”.
Thanks, that makes sense. I read the entry in _Albedo to Zodiac_ and the paragraph and illustration are helpful. I shoulda thought of that before posting the question.
As Mars moves up the ecliptic toward the Pleiades and the Hyades, it has passed through a rather barren section of the zodiac, but in recent days I have observed that it makes an interesting and almost symmetrical pattern with neighboring stars that I’ve never noticed before: Alpha and Beta Arietis to Mars’ right, and Alpha and Gamma Ceti on its left, combine with Mars to make a shallow crown. My skies have been light polluted and hazy enough to reduce the visibility of other stars just enough to make those 4 more prominent and create the pattern. It’s skewed toward Aries since Mars is a bit closer to that side than to the head of Cetus.