I'm trying to explain to my daughter that the Summer Solstice is not a day of the year, but an exact instant in time. My search shows that this will be about 21:43 or 21:44 UTC tonight. To make my point I searched for a more precise time. I figured I could find a time accurate to at least the second, or hopefully several places after the decimal , but I couldn't find anything.

Can anyone show me a source for the precise time of the Solstice to at least the second? To the millisecond? The microsecond?

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    $\begingroup$ I'm coming from the field of engineering, not from the field of astronomy: The motion of objects is never an ideal motion. The earth for example is hit by meteorites influencing the motion every day. This will cause the axis of rotating objects to wiggle a bit. This wiggeling makes it hard to define which point in time exactly the "solstice" is - even if the exact motion is known. $\endgroup$ – Martin Rosenau Jun 21 '20 at 7:47
  • $\begingroup$ @MartinRosenau That's a reasonable guess, but the momentum of the meteor material is pretty tiny compared to the angular momentum of the Earth. Larger effects come from things like angular momentum "sloshing" back & forth between the atmosphere, oceans, and the (not so) solid body of the Earth. $\endgroup$ – PM 2Ring Jun 21 '20 at 10:33
  • $\begingroup$ From the IERS: The variability of the earth-rotation vector relative to the body of the planet or in inertial space is caused by the gravitational torque exerted by the Moon, Sun and planets, displacements of matter in different parts of the planet and other excitation mechanisms. $\endgroup$ – PM 2Ring Jun 21 '20 at 10:36
  • $\begingroup$ (cont) The observed oscillations can be interpreted in terms of mantle elasticity, earth flattening, structure and properties of the core-mantle boundary, rheology of the core, underground water, oceanic variability, and atmospheric variability on time scales of weather or climate. The understanding of the coupling between the various layers of our planet is also a key aspect of this research. $\endgroup$ – PM 2Ring Jun 21 '20 at 10:36
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    $\begingroup$ @PM2Ring I was aware of that. I mentioned the meteorites only because it is obvious that this impact cannot be calculated while it is less obvious that the influence of the moon on the liquid interior of the earth is difficult to calculate. Important is: All these influences make it impossible to define an "exact" time of the solstice. The question is: How precise (minutes, seconds, milliseconds) can it be defined? $\endgroup$ – Martin Rosenau Jun 21 '20 at 10:59


In a couple of weeks it might be possible to say precisely (within a second, perhaps a fraction of a second) when the summer solstice did occur. But until then, sub-minute estimates should be treated as fraudulent.

The reason you are seeing different times for when the summer solstice will occur is that different websites use different models of the orbits of the bodies that comprise the solar system and different models of the Earth's orientation in space.

It's very important to remember that "all models are wrong, but some are useful".

There are several issues with finding the exact time of when solstices occur. One is that the solstices are extrema of the Sun's declination. Without a model of derivatives, finding extrema (e.g., the solstices) mathematically is a much harder problem than is finding zero crossings (e.g., the equinoxes).

More importantly, a very precise prediction would require multiple very precise models. While many models of the orbits of the bodies in the solar system are very, very good, none are good enough to get the timing of the solstices down to the millisecond. Even more importantly, the tilt of the Earth's orientation in space with respect to its orbit about the Sun needs to be modeled. The best model, the IAU 2006/2000A precession-nutation model has over a thousand terms (1365 terms, to be precise).

And even then, "all models are wrong." The people at the International Earth Rotation and Reference Systems Service (IERS) know this very well. The IERS is the organization that is responsible for modeling time as measured by the Earth's rotation and for modeling how the Earth is oriented in space. There are some aspects of the Earth's orientation in space that are not quite predictable. The US Naval Observatory, one of the key members of the IERS, publishes daily updates of observed deviations from model predictions. It take a couple of weeks to fully digest measurements made by astronomers worldwide.

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    $\begingroup$ What was the exact time of the last two solstices? $\endgroup$ – d-b Jun 21 '20 at 10:35
  • $\begingroup$ You mean the last 2 summer solstices or just the last 2 solstices (including winter solstice)? (You can google it though for the time) $\endgroup$ – Huy Pham Jun 22 '20 at 18:17

It is intrinsically hard to measure the exact time of solstice as, unlike the equinox, it occurs when the sun's declination is changing slowly. So, determining the exact time of solstice depends on models of nutation, etc.

timeanddate.com has a user-created countdown time that uses 21:43:40 UTC for the exact time of solstice. I'm a little sceptical, as they use 21:43 elsewhere on their site.

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    $\begingroup$ The 40 seconds would explain why some sites have 21:43 (truncated or rounded down) and some have 21:44 (rounded up) according to OP. $\endgroup$ – JohnHoltz Jun 20 '20 at 17:59
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    $\begingroup$ @JohnHoltz - Some have 21:42, and wolfram alpha has 21:30. "All models are wrong, but some are useful." $\endgroup$ – David Hammen Jun 20 '20 at 19:01
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    $\begingroup$ My calculation from DE-438t is 21:43:39.66694 UTC $\endgroup$ – Huy Pham Jun 21 '20 at 2:56
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    $\begingroup$ @HuyPham, that is probably worthy of an answer, if you can show how you used DS-438t $\endgroup$ – James K Jun 21 '20 at 14:38
  • $\begingroup$ To simplify, from DE-438t you can get the Sun position at a date.... and Summer Solstice means the Sun Longitude is equal to 90 degrees... (after that, applied precession/nutation)... $\endgroup$ – Huy Pham Jun 22 '20 at 5:56

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