Searching for the Double Diamond Ring

Baily’s beads seen 2 July 2019 from CTIO. Credit: CTIO/NOIRLab/NSF/AURA/D. Munizaga

What’s a Double Diamond Ring?

A picture being worth a thousand words, you probably already have a pretty good idea of what the double diamond ring is from the image above!

In certain circumstances, during a solar eclipse, instead of the light of the Sun diminishing to a single point - the classic “ Diamond Ring ” - it lingers in two places giving the elusive “ Double Diamond Ring ” effect. As totality arrives, a true double diamond converges on just two points of light, at least as perceived by human eyes and most cameras.

This is captured perfectly in the wonderful image above published on the NOIRLab website and kindly made available for publication via a Creative Commons license.

The Double Diamond is a special case of Baily’s Beads, the phenomenon whereby the last (or first) rays of the Sun’s photosphere remain visible through lunar valleys around the moments of second and third contact.

What’s going on in this case?

The image above was photographed during the 2 July 2019 total solar eclipse. The location was the Cerro Tololo Inter-American Observatory north of Santiago, Chile.

The observatory lies around half way between the central path and the southern limit of the eclipse:

CTIO observatory shown on the path of the 2 July 2019 eclipse, (source)

Using the Photo Ephemeris eclipse simulator, we can check the lunar limb profile. It’s immediately clear why we’re seeing a double diamond in this case:

Double Diamond before C2 during 2 July 2019 eclipse, as viewed from CTIO

The orange exaggerated relief profile of the lunar limb shows two broad lunar valleys corresponding with the position of the double diamond. It’s exactly this sort of profile that can give rise to the effect. It requires two valleys and that they not be separated by too great an angle for the magnitude of the eclipse - it’s around 30° in this case.

If the greatest magnitude of the eclipse is very close to 1.000, then the separation can be any value, but for larger magnitudes the greater relative apparent size of the Moon’s disc means that the valleys need to be closer. If too far apart, the movement of the Moon relative to the Sun means the double diamond is geometrically impossible.

In the case of the July 2019 total eclipse, the greatest magnitude at CTIO was 1.0074 with standard totality lasting 2 minutes 3.7 seconds (without lunar limb corrections - it was around 4s less than that in fact due to lunar valleys).

Overlaying the simulator on the image, you can see that the alignment is excellent. The only adjustment required was to rotate the image from the simulator by 4° to match the rotation of the camera relative to the horizon:

How can I view a Double Diamond on 8 April 2024?

The rule of thumb is that to maximize the duration of Baily’s beads, you should observe from near the edges of the central path. You typically head towards the northern limit for an annular eclipse and the southern limit of a total eclipse. Why? Because the southern pole (the 6 o’clock region) of the Moon is rougher than the northern pole, providing more hills and valleys in which beads may form. During an annular eclipse, the antumbra of the Moon means the southern limb grazes the Sun’s limb when viewed from the northern edge of the path.

So what about April 8 2024? Let’s pick a rough starting point for our explorations - say Uvalde, Texas. Just like CTIO in 2019, Uvalde lies some way south of the central path. However, that’s where the similarities end for the purposes of a double diamond ring.

The libration of the Moon has changed this time around, meaning we see a different edge profile than in 2019. The two broad valleys from July 2019 are not visible in the same way in Uvalde on 8 April 2024, and so a double diamond cannot form. Additionally, the greatest magnitude is higher than at CTIO in 2019, 1.0199, meaning the valleys would need to be closer together. There’s certainly some mathematical relationship that could be derived here, but I haven’t worked it out yet!

Looking at the contacts vs the limb profile, it’s not going to give rise to a double diamond ring:

There are two other areas of the limb that look more promising:

Uvalde’s C2 position angle appears a long way from the two likely candidates, so we are going to focus on C3 instead. Can we find a nearby location for which third contact occurs in one of our target zones?

Exploring the map and spot checking a few different locations, I came up with this, east of Uvalde and a little northwest of Devine, Texas:

April 8 2024, near Devine TX: a brief double diamond at C3, source

The simulator shows the double diamond lasting around 3 seconds, starting just before the standard C3 time of 1:34:02.4pm. Short, but sweet.

Now you know what to look out for, have an explore and see if you can find a double diamond spot for the next eclipse!