How to map a Moon shadow: why plotting the Path of Totality was a bumpy ride

“The knowledge of the true lunar limb profile is a major breakthrough” says eclipse mapper Xavier Jubier

If you’ve done any kind of planning for the Total Solar Eclipse on August 21, chances are you’ve already found these Google Maps and Google Earth files devised by eclipse mapper Xavier Jubier.

You can use them to find out exactly where to stand on August 21. They show the part of the Moon’s shadow where the entire Sun is blocked by the moon – the umbra – in exacting detail. But while it shows you where to get to in order to experience Totality, it does so in awesome detail.

While the borders of that shadow – the edges of the Path of Totality – were once shown as smooth, now they’re incredibly nuanced. So how on Earth (literally) are they now so accurate? Every valley and mountain on the Moon can now be translated into an exact shadow shape on the planet 240,000 miles/385,000 km below. This is the lunar limb, the edge of the visible surface of the Moon as viewed from Earth.

“What you see is going to depend on your location,” says Bob Baer, staff member at SIU Carbondale Physics Department, which is right on the centerline on August 21.

“Xavier Jubier has these great predictions – he’s now able to predict Baily’s beads in advance because of the highly accurate lunar limb data he’s using in his maps.”

Eclipse calculations used to assume that all observers are at sea level, and that the Moon is a smooth sphere (cue some eclipse-chasers missing Totality even though the map indicated they were inside the Path of Totality), but that’s no longer the case. “The knowledge of the true lunar limb profile is a major breakthrough, first with Kaguya in 2009 and later the Lunar Reconnaissance Orbiter (LRO),” Jubier told

Kaguya – named after a mythical Japanese Princess that visited Earth from the Moon – was a lunar orbiter launched by the Japan Aerospace Exploration Agency (JAXA). It took incredible HD video of the Moon and Earth. NASA’s LRO is a remote sensing platform that is now orbiting the Moon, and has been since 2009. Both collected 3D limb profile data so exact that the precision of eclipse contact times and durations of Totality can now be calculated down to less than a 0.2 second level of accuracy – as indicated in Jubier’s Google Maps if you click anywhere.

But the Moon’s topography is only half the story. “There is much more to it because regular maps are drawn at sea level, but the local terrain elevation has to be taken into account as well,” said Jubier. Put simply, the elevation you stand at to view Totality will affect what you see – particularly if you stand at the edge.

In fact, if you do stand at the edge – which remains a risky business in terms of whether you see Totality or not – the mountains on the Moon start to really matter.

So now for the big question: where is he going to stand come August 21? “Definitely on the ground in eastern Idaho, unless the weather forecast dictates otherwise,” said Jubier, citing the the probability of blue sky in the western part of the eclipse path. That’s some confidence from someone who frequently watches eclipses while airborne. “Doing an eclipse flight just to get more duration doesn’t really make sense unless some good scientific work can be done,” he adds.

“That said, I may have a private jet ready should the weather be really bad in a 500 miles radius.”

Well, why not? After all, good eclipse-chasers always have a Plan B.

Photo credit: JAXA/NHK