The very beautiful Jupiter Venus conjunction of ~ 2019-01-20 (practically something out of "2001 A Space Odyssey") is seen as "vertically aligned" in the dawn sky, Venus "above" Jupiter.

See this picture by Jeff Majewski at EarthSky.com to see what I mean (not sure I can copy it in here directly)

This should mean my "vertical" is aligned with the Ecliptic. I'm in northern latitudes (49° N), how is that possible?

And is this alignment geometrically linked with the "Super Wolf Blood Moon" which was very visible at the same time?


The problem seems hard to explain so here is a little free-hand drawing.

free hand drawing of observation by an observer standing on the equator

The observer sits on the equator and is just being moved into sunlight by Earth's rotation (the image shows Winter Time for the Northern Hemisphere). The observer's "UP" vector lies in the Ecliptic Plane. This being so, and Jupiter and Venus also being in the Ecliptic Plane, the observer can see Jupiter and Venus aligned "vertically" (same azimuth), separated by a couple of degrees in elevation. As the day progresses, that vertical alignment will evidently cease as the line connecting the planets rotates relative to the observer.

This works on the Equator, but what if the observer is at higher latitudes? I can't get my head around the idea that the planets can be vertically aligned while still being on the Ecliptic Plane.

  • $\begingroup$ Isn't just the difference in orbital inclination? Anyway I will try to spot it as well :) $\endgroup$
    – Alchimista
    Jan 21 '19 at 11:58
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    $\begingroup$ Nice drawing, showing a very good understanding of the geometry involved in your question. However, as mentioned in my answer, the planets are not exactly on the ecliptic plane, so a vertical alignment can be seen from other places too. $\endgroup$
    – FSimardGIS
    Jan 21 '19 at 22:47
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    $\begingroup$ @FSimardGIS Yes, that was the assumption I made which had to be relaxed to find the real world-world solution. Excellent answer. One immediately gets the orbital racetrack feeling. $\endgroup$ Jan 21 '19 at 22:53
  • $\begingroup$ "Super Wolf Blood Moon" - that's actually a Moon eclipse. The "wolf", "blood" and other such teenage fantasy vampire book terms are just how the social media hashtags described it. $\endgroup$ Jan 22 '19 at 11:26

A combination of several movements can lead two objects in the sky to appear vertically aligned. The planets are constantly moving relative to each other, they are orbiting on slightly different planes, the Earth is orbiting and rotating, so such configurations between planets can occur frequently.

An observer does not need to be in the tropics to see two planets being vertically aligned at some point. The planets orbit close to the ecliptic, but their orbital planes are slightly inclined relative to ours. Jupiter's orbit is about 1° from the ecliptic, and Venus is about 3°. This means that both planets can move above or below the ecliptic by a few degrees. When they pass each other as seen from the Earth in a conjunction, they are rarely on top of each other, most of the time, they are separated by a few degrees, and they are not exactly on the ecliptic line. (Such occurences of planets occulting or transiting each other do happen, but are rather uncommon: the next time Venus will transit in front of Jupiter will be on 22 November 2065)

I'll add a few pictures to help visualizing the positions, movements and alignments involved here.

Here is a view of the sky on 20 January at 49° North latitude:

enter image description here

As you can see, Jupiter and Venus appear pretty much vertically aligned in the morning sky. The yellow line is the ecliptic. Note that the ecliptic does not pass through the planets here. Jupiter is pretty close, but Venus is a few degrees north of it. As both planets pass each other in the next days, at some point, their alignment will be perpendicular to the ecliptic (called the ecliptic conjunction), as we can see from this January 22 simulated image:

enter image description here

But they won't appear vertical anymore from your vantage point. (In fact, they would apprear vertical again later during that day, because of Earth's rotation, but the Sun will be up, so it wouldn't be observable directly)

A few days later, on January 26, they will appear horizontally aligned!

enter image description here

(Images generated in Stellarium)


I'm even further north than you and it was vertically aligned for me at one point through that night as well, as the night sky turned

Your latitude is not really relevant. It only really matters at what time you observed it. I'm trying to find a useful picture sequence showing them moving overhead - I don't have one unfortunately, so perhaps try and imagine Orion, for example. It rotates through the sky overnight, so when Orion rises, his belt is almost vertical, with Mintaka being higher in the sky. By the time Orion is setting, Alnitak is uppermost in the belt.

And it has absolutely nothing to do with the lunar eclipse :-)

  • $\begingroup$ As I see it, it means the "vertical vector" is at some point in a plane parallel to the ecliptic. For me, this currently happens to be sunrise. For (90-23.5)° N it would be midnight, at which point neither Jupiter nor Venus would be visible. For anything at (90-23.5)° N to 90° N, Jupiter and Venus will never be "vertically aligned"..? $\endgroup$ Jan 21 '19 at 13:00
  • $\begingroup$ I'm not sure I understand you. Jupiter and Venus move past each other in the sky once a year. So you can choose when you see them vertically. I see vertical as relative to the earth, not relative to the ecliptic (but even if it were, surely it should be perpendicular to the ecliptic :-) $\endgroup$
    – Rory Alsop
    Jan 21 '19 at 14:00

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