(The two little crescents in the above picture are the planets Jupiter and Venus. Their shape is an artifact of my camera and blowing up the picture beyond what it can manage.)
There are two definitions of the ecliptic that are easy to find. One says that the ecliptic is the plane of the earth’s orbit around the sun. Imagine a flat disk with a hole in the center where the sun is. The edge of this disk would be where the earth orbits. This is not, of course, the same as a disk stretching out from the earth’s equator. That imaginary disk is tilted.
The second definition says that the ecliptic is the apparent path of the sun across the heavens. … that road of the sun where Phaethon couldn’t control his chariot … We know that the earth is turning on its axis and that this causes the sun to appear in the east every morning and disappear in the west every evening. But to our normal senses the sun moves and we stand still. And the sun stays within a very small band in the sky. The other planets also stay very close to that same band, as does the moon.
“Very close” is not the same as identical, by the way. Only when the the path of the moon actually crosses the ecliptic plane can there be an eclipse. And yes, that is where the name came from.
Dante had no trouble with this idea but we do not pay as much attention to the sky as the medievals did. But over the past month as Jupiter and Venus have appeared in the western sky after sunset it has been easy to follow the wide band in the sky where the sun and planets make their appearance. As you can see in these pictures the planets are nearly in a straight line. They are nearly in line with the moon. They are very close to the path of the sun.
In this picture taken by Tony Meyerhofer we see at the top a crescent moon (easier before the picture was blown up), then Jupiter, then Venus. It was taken through the trees outside our house. This was around February 23.
In this picture, a few days later the planets are closer together though Venus is still below Jupiter. The waxing moon is high overhead and has brightened the sky.
Taken just after sunset (probably on February 28) you can see the two planets, Jupiter and Venus, caught in the trees just above the middle of this picture. You can also see that roughly speaking the alignment of the planets points to the brightest part of the sunset below. This picture is later than the first one because the moon is not visible. It is high above.
As Jupiter and Venus reached their closest approach the sideways distance between them became apparent. But afterwards they went back to looking quite linear. And after I had watched these planets for a couple of weeks I could SEE the ecliptic in a way I had never done before. Diagrams drawn to show the ecliptic are often done as though the observer is looking down on the solar system from above the North Pole. This really complicates the geometry of what is actually seen.
So here are some extra APOD pictures to look up for more entertainment.
This lovely picture of the planets, Jupiter and Venus, and the moon features the occultation of Jupiter, briefly hidden, as it passes behind the moon. See the line the three celestial objects are making. https://apod.nasa.gov/apod/ap230225.html
This picture includes the zodiacal light along with an explanation of it as Jupiter’s satellites producing dust that spirals in towards the sun. https://apod.nasa.gov/apod/ap230227.html
The closest approach of Jupiter and Venus is shown over a lovely landscape in Sicily. https://apod.nasa.gov/apod/ap230305.html (The crescent in the upper right is a trademark NOT the real moon.)
https://apod.nasa.gov/apod/ap230304.html This picture consists of ten panels showing the position of Jupiter and Venus for ten days. Strip #2 includes the crescent moon which looks as if it is next to the planets. But if you were looking up into the whole sky you would see how close together all these celestial objects are. Jupiter began high in the sky above Venus. The two planets were ‘next’ to each other on March 1 and now Jupiter is below Venus, while Venus has risen high in the sky and is very bright after sunset.
Dante describes several different heavenly arcs. This arc is the line of light and dark that runs around the earth, pole to pole.
To that horizon had the sun now come,
an arc that circles both the hemispheres,
whose zenith stands above Jerusalem …
And Night below, in circling the same way,
rose from the Ganges …
Purgatorio, Canto 2, Esolen
Here Virgil is discussing why Dante sees the sun on his left as he looks at the dawn. He is in the Southern Hemisphere. Virgil includes the idea that the sun’s shifts north or south with changing seasons.
… [Virgil] remarked, “now if the Gemini
were in the company of the flashing mirror (the sun)
which lights from dawn to dusk across the sky,
You’d see that firestar of the zodiac (the sun again)
wheel even tighter … “
Purgatorio, Canto 4, Esolen
And one more quote from Paradise, Canto 29, Longfellow. The children of Latona are the sun and the moon, which with various constellations, are forming a zone or band around the horizon. The moon in this case is full and therefore opposite the sun. When the sun sets and the moon rises they “change their hemisphere” and Beatrice has been silent for these few moments.
At what time both the children of Latona,
Surmounted by the Ram and by the Scales,
Together make a zone of the horizon,
As long as from the time the zenith holds them
In equipoise, till from that girdle both
Changing their hemisphere disturb the balance,
So long … did Beatrice keep silence …
4 thoughts on “Jupiter, Venus, and the ecliptic”
This is one of the best explanations of the ecliptic I have seen. It is without doubt the most beautiful one.
Here is some more celestial geometry.
our Lady of Guadelupe is standing on a crescent Moon, we all remember: https://adw.org/wp-content/uploads/sites/2/2018/11/OLG-Explained.pdf .
To see what the phase of the Moon really was, we can use Horizons, https://ssd.jpl.nasa.gov/horizons/app.html#/
where we will query about an Observer table for Luna, in a Geocentric frame of reference, from 1531-01-01 to 1531-12-31, and under Table Settings we say that the Calendar Type must be Gregorian, and then we Generate Ephemeris, and then we look at the phase of the Moon as Ilu%. Now to find the date Dec 12, 1531, which was a Julian date, we look here in Horizons for December 22, 1531, we find that New Moon was about December 19 and so we are a few days past New.
So the phase of the Moon on 12 December 1531 (in Julian calendar) was a few days past New. It was a waxing crescent. And if we look up a waxing crescent, we see that there should be Mare Crisium and a bit of Mare Fecundatis.
And that does correspond in my biased opinion to the depiction of the crescent Moon that Our Lady of Guadelupe is standing on….. with that kind of oval blot there on the right side. In my biased opinion, that blot looks like Mare Crisium.
So the Moon she is standing on is just the Moon of that very date.
I am biased, but have done my best, and you see what you think.
Another biased opinion, for which I invite correction, is that the details of the crescent Moon 3 days old, would be very hard to see, because in this phase the Moon is visible against a dark sky only for a few hours after sunset, and it is low in the sky at the time.I am not really sure anyone without a telescope would have been able to toss off a drawing of a waxing crescent moon that did an essentially correct job of depicting Mare Crisium.
But I am very biased.
And one more thing – on the very edge of the Moon as painted on the tilma, there is another of the lunr maria apparent. That appears to be in position to be Mare Marginis. Of interest, Mare Marginis is not always visible – it depends on the libration of the Moon. And what was the libration of the Moon on (Gregorian calendar Dec 22, 1531 or Julian calendar Dec 12, 1531) ?
Love the pictures!