Happy Thought for 21 June

Have a Happy Thought:

 We have passed the solstice – halfway through the solar year – at 8:51am on Friday 21^st June, New Zealand time – that is, a few hours ago.

In most years, the solstice hits on the calendar date of June 22^nd in New Zealand. But this year, it’s a bit earlier.

 A couple of definitions/explanations first:

1. The solstice occurs when the Earth’s poles are most directly pointed toward (or away) from the sun. Because it’s the North Pole that’s pointed toward the sun right now, it’s summer solstice in the northern hemisphere. And winter solstice in the southern hemisphere, because we’re pointed away from the sun.
2. The solstice is when you get the longest night and shortest day of the year (winter)… or vice versa (summer).
3. Equinox (think: equal) is midway in between the winter and summer solstices. So they happen in the midst of spring and autumn, and tend to have equal amounts of day and night.

 So back to the reasons why the solstice is earlier-than-normal this (calendar) year:

 The first is the most obvious: it’s a leap year! We stuffed an extra day into February, so that pushes us a day ‘later’ than normal.

 But it turns out that leap minutes and seconds matter here, too. We have leap years because it actually takes just over 365 days for the Earth to go around the sun. So a leap year every four years assumes that it actually takes us 365.25 days. But of course, orbital mechanics and planetary rotations are not that precise, or simple. It actually takes us closer to 365.242189 days to go around the sun. This means that we’re adding just a bit too much every four years, so the solstice actually gets “earlier” by 45 minutes every four years when we do the leap-day correction… at least until we get to 2100.

 Because every century (1800, 1900, 2100, etc) we skip the leap day. [Unless it’s also divisible by 4, in which case we still need the leap day (2000).]

 Which means that 2096, the last leap year before we re-set for another century, will have the earliest solstice in.. a really long time. The western parts of Turtle Island (North America), for example, will actually experience the “moment” of solstice on June 19^th!

Solstice dates in Vancouver, British Columbia, Canada, classified by “type” (the remainder after dividing the year by 4). Years evenly divisible by 4 (type 0 – such as 2016, 2020, 2024) can have early solstices. Credit: Milan B/RASC Vancouver

 

But of course it all gets even more complicated because the Earth’s rotational speed (length of a day) changes. It does this quite a bit, actually! A lot of the variability is down to tides (so yes, “time and tides” are very much connected!), since tides are really the effect of gravity from the sun and the moon, tugging the Earth around a bit.

Some of the variability is also down to how the Earth’s core is spinning – and we can figure this out by watching changes in the Earth’s magnetic fields.

 A new contribution is down to us (humans). Climate change is melting glaciers, we all know this. A lot of that ice is melting from near the poles, and the water ends up moving toward the equator. This is then your typical “figure skater doing a spin, pushing her arms out and slowing down in the spin” situation. This slowdown has been happening since 1972, and by 2029 it’ll have added up to a whole second – hence the possible need for a negative leap-second at some point in the near future.

 Adding up those last two reasons leads to some beautiful phrasing in the article about this, linked below:

Recent changes in spin caused by ice and iron

 Which is a phrase that will ring in my head for a while… maybe until the next leap year?

In any case:

If you’re in the Northern Hemisphere, get out and enjoy those long days (stick to morning and evening if you’re in a heat wave area though!!)

If you’re in the Southern Hemisphere: the longest dark has passed, and summer is coming.

  

You can read more about leap years and days and why they do this 4, 100, and 400 year cycle here: https://bigthink.com/starts-with-a-bang/summer-solstice-earliest-since-1796/

 

And more about why we might need a negative leap second, here: https://www.nature.com/articles/s41586-024-07170-0