Tag Archives: gravity

The zero meridian, or something like it

CIMG1443This weekend I took my parents to visit the Greenwich Meridian – or did I?

The marked meridian on the site of Greenwich Observatory, where tourists line up to pose for silly pictures with one foot in the East and one foot in the West, has claimed to be zero degrees longtidude since 1884, but if you check your smart phone GPS on that spot, you’re NOT at exactly 0.000 degrees.

According to GPS, the zero meridian appears to be in a park adjacent to the observatory, and not in the section behind the fence that charges admission so you can “visit the meridian”.

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What’s going on here?

Earlier this month, an article by Stephen Malys and others in the Journal of Geodesy revealed the reason behind the discrepancy. The technology used in the 19th century to determine the location of the zero meridian was subject to local distortions from the Earth’s gravity and shape of the local terrain. GPS technology uses measurements from satellites, which aren’t affected in the same way as technology located on Earth.

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The dotted line is the much photographed meridian established in 1884. The solid line is where the GPS says it should be.

So the meridian really is in the wrong place. What does that mean for maps or for time? Well, the Ordnance Maps used in the UK were already using a slightly different zero meridian as reference point, because they were established before the 1884 meridian convention. And the effect of the new meridian location on Greenwich Mean Time, which determines Universal Time, is unnoticably small, so nothing much has changed.

Except, for a shorter line and a cheaper visit, you could technically skip the museum and the crowd of tourists and find the true GPS meridian about a hundred meters to the East of the Observatory in Greenwich Park. It’s probably not as fun a place for a family visit, though.

Aerial photo is Figure 1 from the article by Malys et al. (CC-BY). Photos taken from the ground are by me and by the man who was behind us in line at the almost-but-not-quite meridian line. I previously wrote about the history of the Greenwich Observatory on this site.

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Nuttier

16280779113_1f69bb9871_oLast week I posted this photo of an animation that was used regularly during the NCAA Men’s Basketball Tournament and asked you to divine which of the multiple problems* bothered me so much. I think I’ve left you on pins and needles long enough. Continue reading

Tortoise vs Hare, but in a vacuum this time

We all know how gravity is supposed to work. Without air resistance, a feather and a bowling ball (the standardized materials for all gravitational tests) should accelerate toward the center of the Earth at the same rate, thus striking the ground at the same time. Humans have tested this. It works.

Although we know this thing, it is so far removed from our daily experience that it is still stunning to watch it happen. This fundamental principle is nicely illustrated in this video from the BBC. The video also nicely shows how amazed a roomful of individuals who know how the experiment will work can be when the experiment works exactly as expected.

That is why we need the scientific method to rigorously test hypotheses and incrementally build our knowledge of how the universe works. Our day-to-day experience of and intuition about the world is extremely valuable, but also extremely deceptive.

For the record, the tortoise vs hare in a vacuum race I alluded to in the title would be incredibly inhumane and disappointing, in addition to having no winner – unless, UNLESS we had the tortoise and hare race in spacesuits. Why aren’t we racing animals in spacesuits?

HT: Jared Heidinger