Happy solstice
The sun is coming back! This is the bottom of the year.
Except it isn't quite. The days do start to get longer, but very slowly. We are tilting back towards the sun, but again very slowly. It'll be dark and cold for months yet. (Although this year it hasn't been very cold so far.)
There is some confusion about when the solstice is and what it is. So here's my understanding.
Note: All this assumes you're in the Northern hemisphere, some way away from the equator. You'll have to stand things on their heads for the Southern hemisphere. Also, this is posted in get-it-out-quick mode so may have errors and lacks links.
The Earth is a sphere (actually a very-slightly flattened sphere, which makes an oblate spheroid) that spins on its axis once a day. This is what gives us day and night.
Digression 1: A day as timed from noon to noon (sun due South) is called a solar day, and is really pretty close to being 24 hours, because that's how we defined seconds - but it's not quite perfect, because even if we'd measured it accurately enough, the Earth's speed of rotation changes over time. In essence, our clocks can keep time much more accurately than the Earth does. We currently deal with the misalignment this causes by leap seconds. Anyway, a solar day pretty damn close to 24 hours. You can also time a day by the distant stars, which is called a sidereal day. This is very slightly shorter than a solar day! That's because the solar day has to account for the Earth's orbit round the sun: by the time the Earth has made one complete rotation (a sidereal day), it has also moved about 1/365th of the way round its orbit, so it needs to turn a little bit more for the same bit to be pointing towards the sun as when it started. The difference between sidereal and solar days is about four minutes, which can be useful for things like spotting whether a periodic signal is coming from the Earth or from space.)
The Earth orbits round the sun once a year.
Digression 2: Unfortunately, this doesn't take a whole number of days: it takes a bit less than 365.25 days. We deal with this misalignment by leap years: once every four years we add an extra day. That averages 365.25, which is slightly too much, so to get it very, very close, we don't have a leap year every hundred years (2100 won't be a leap year), except we do every four hundred years (2000 was a leap year).)
If you imagine the axis of the Earth's rotation extended in to the sky, the place it's pointing is called the celestial pole. The North celestial pole is pretty close to Polaris, the pole star. If you imagine the Earth's equator projected out in to the sky, that's called the celestial equator. The Earth's rotational axis is perpendicular to the celestial equator.
The path the Sun appears to follow in the sky is called the ecliptic. (The planets roughly follow this line in the sky too, because most of the Solar System is roughly in the ecliptic plane - the Earth's orbit round the Sun is roughly in the same plane as the other planets' orbits.)
Now, we all know that the Earth is tilted. The axis is about 23.4 degrees tilted away from the ecliptic.
This is what gives us seasons: in winter, the North celestial pole is pointing away from the sun, so we get less daylight, and what sunshine there is falls more obliquely, so it's less warm. In summer, the North celestial pole is pointing towards the sun, so we get more daylight and more warmth.
So some people like to say "axial tilt is the reason for the season", as a smartarse riposte to "Jesus is the reason for the season". I prefer to think of the obliquity of the ecliptic.
The what? The celestial equator (the projection of the Earth's equator on to the sky) is 23.4 degrees oblique to the ecliptic (the path of the sun on the sky). This is called the obliquity of the ecliptic. Wikipedia insists on talking about axial tilt but all the astronomy stuff I've ever read talks about the obliquity of the ecliptic so that's how my brain has it. It does, of course, come to the same thing.
Which is that you have two great circles in the sky: one, the celestial equator, is the Earth's equator projected out; the other, the ecliptic, is the path of the sun. They're 23.4 degrees apart. This means that they cross at two points. Those are the equinoxes!
An equinox, astronomically defined, is the point at which the sun crosses the celestial equator. So, if you do your sums and observations very carefully, you can see that it happens at a precise moment in time. It also happens at a precise place in the sky. So when an astronomer talks about an equinox, they might mean the point in the sky, or they might mean the moment in time when the sun is at that precise point in the sky.
(Those two places in the sky move very slightly, coming back to where they started after 26,000 years. This is because the Earth is wobbling a bit as it spins. The wobbling is called 'precession', and so this is called the 'precession of the equinoxes'. Wikipedia again insists that this is called axial precession these days and only gets called precession of the equinoxes "when detailed mathematics are absent", but when I did the maths it was bloody well was called the precession of the equinoxes. And it sounds cooler.)
Culturally, though, the equinox is the time of equal length night and day. Indeed, that's the origin of the name.
This is not quite right: it's equal night and day if there the sun and the Earth were single points in space, and there were no atmosphere. But in reality, the Earth is far from being a single point, and the sun is really quite big indeed [citation needed]. So we can see the sun for quite some time before the centre point of it rises above the horizon. Likewise, we can see some of it for quite a while after the centre point has set. Even better, the atmosphere bends the sun's light so we see the tip of the sun rising slightly before it is geometrically above the horizon. So even when they're perfectly balanced, we actually get more sunlight than dark! Even better, we get twilight as well. The sun lights up the atmosphere above us long before it rises and long after it sets.
I really like this symbolically: there is a fundamental skew in favour of the light.
Ok, so equinoxes are when the sun crosses the celestial equator. What about solstices? Well, they are the point at which the sun is furthest away from the celestial equator, roughly half-way between the equinoxes.
Again, astronomically, a solstice is both the point in the sky where the sun is furthest from the celestial equator, and the moment in time when the sun is at that point.
The solstices are thus the point in time when the axis of the Earth is tilted furthest (for winter) or nearest (for summer) to the sun.
The winter solstice is also when the sun appears lowest in the sky at its highest point (solar noon). Before the solstice, it's been getting lower and lower, and after, it gets higher. At the precise moment of the solstice, it stands still - hence solstice (from Latin solstitium, sol=sun, stit-= stationary). The summer solstice is when the sun appears highest in the sky, and the sun starts to move lower. The precise moment of the solstice is only solar noon for a single line of longitude on the planet, which is almost certainly not the one you are standing on. But the solar noon nearest to the solstice will be the maximum or minimum you'll see.
The day in which the winter solstice falls is also the shortest, except for some complexity about when you count the start of a day. The times of sunrise and sunset don't quite behave as you would naively expect them to round the solstices either, because of the difference between actual solar time (where the sun is in the sky) and mean solar time (the time we keep on watches). This has another great name: the equation of time. But that's yet another explanation and this post is already too long.
Ok, so an equinox is when the sun crosses the celestial equator, and a solstice is when it's furthest from the celestial equator. But when is that in calendar terms? Well, this gets a little complicated because, as established in the Digressions above, days and years don't line up perfectly. On average the fun and games with leap years keeps the solstices and equinoxes very close to the right points in the calendar, but it isn't perfectly right every year. The solstices are about 21 June and 21 December, but they can be a day either way. The only way to be sure is to look it up.
Well, the only way to be really sure is to do the observations and calculations yourself. I did do a load of this, aaaages ago, and got the same answers you get all over the place. So I say you can trust the obvious reliable sources. The winter solstice this year was at 04:48 GMT this morning, 22 December 2015.
Happy solstice! More light!
This entry crossposted to http://doug.dreamwidth.org/309240.html, where there are
comment(s) not shown here.
Except it isn't quite. The days do start to get longer, but very slowly. We are tilting back towards the sun, but again very slowly. It'll be dark and cold for months yet. (Although this year it hasn't been very cold so far.)
There is some confusion about when the solstice is and what it is. So here's my understanding.
Note: All this assumes you're in the Northern hemisphere, some way away from the equator. You'll have to stand things on their heads for the Southern hemisphere. Also, this is posted in get-it-out-quick mode so may have errors and lacks links.
The Earth is a sphere (actually a very-slightly flattened sphere, which makes an oblate spheroid) that spins on its axis once a day. This is what gives us day and night.
Digression 1: A day as timed from noon to noon (sun due South) is called a solar day, and is really pretty close to being 24 hours, because that's how we defined seconds - but it's not quite perfect, because even if we'd measured it accurately enough, the Earth's speed of rotation changes over time. In essence, our clocks can keep time much more accurately than the Earth does. We currently deal with the misalignment this causes by leap seconds. Anyway, a solar day pretty damn close to 24 hours. You can also time a day by the distant stars, which is called a sidereal day. This is very slightly shorter than a solar day! That's because the solar day has to account for the Earth's orbit round the sun: by the time the Earth has made one complete rotation (a sidereal day), it has also moved about 1/365th of the way round its orbit, so it needs to turn a little bit more for the same bit to be pointing towards the sun as when it started. The difference between sidereal and solar days is about four minutes, which can be useful for things like spotting whether a periodic signal is coming from the Earth or from space.)
The Earth orbits round the sun once a year.
Digression 2: Unfortunately, this doesn't take a whole number of days: it takes a bit less than 365.25 days. We deal with this misalignment by leap years: once every four years we add an extra day. That averages 365.25, which is slightly too much, so to get it very, very close, we don't have a leap year every hundred years (2100 won't be a leap year), except we do every four hundred years (2000 was a leap year).)
If you imagine the axis of the Earth's rotation extended in to the sky, the place it's pointing is called the celestial pole. The North celestial pole is pretty close to Polaris, the pole star. If you imagine the Earth's equator projected out in to the sky, that's called the celestial equator. The Earth's rotational axis is perpendicular to the celestial equator.
The path the Sun appears to follow in the sky is called the ecliptic. (The planets roughly follow this line in the sky too, because most of the Solar System is roughly in the ecliptic plane - the Earth's orbit round the Sun is roughly in the same plane as the other planets' orbits.)
Now, we all know that the Earth is tilted. The axis is about 23.4 degrees tilted away from the ecliptic.
This is what gives us seasons: in winter, the North celestial pole is pointing away from the sun, so we get less daylight, and what sunshine there is falls more obliquely, so it's less warm. In summer, the North celestial pole is pointing towards the sun, so we get more daylight and more warmth.
So some people like to say "axial tilt is the reason for the season", as a smartarse riposte to "Jesus is the reason for the season". I prefer to think of the obliquity of the ecliptic.
The what? The celestial equator (the projection of the Earth's equator on to the sky) is 23.4 degrees oblique to the ecliptic (the path of the sun on the sky). This is called the obliquity of the ecliptic. Wikipedia insists on talking about axial tilt but all the astronomy stuff I've ever read talks about the obliquity of the ecliptic so that's how my brain has it. It does, of course, come to the same thing.
Which is that you have two great circles in the sky: one, the celestial equator, is the Earth's equator projected out; the other, the ecliptic, is the path of the sun. They're 23.4 degrees apart. This means that they cross at two points. Those are the equinoxes!
An equinox, astronomically defined, is the point at which the sun crosses the celestial equator. So, if you do your sums and observations very carefully, you can see that it happens at a precise moment in time. It also happens at a precise place in the sky. So when an astronomer talks about an equinox, they might mean the point in the sky, or they might mean the moment in time when the sun is at that precise point in the sky.
(Those two places in the sky move very slightly, coming back to where they started after 26,000 years. This is because the Earth is wobbling a bit as it spins. The wobbling is called 'precession', and so this is called the 'precession of the equinoxes'. Wikipedia again insists that this is called axial precession these days and only gets called precession of the equinoxes "when detailed mathematics are absent", but when I did the maths it was bloody well was called the precession of the equinoxes. And it sounds cooler.)
Culturally, though, the equinox is the time of equal length night and day. Indeed, that's the origin of the name.
This is not quite right: it's equal night and day if there the sun and the Earth were single points in space, and there were no atmosphere. But in reality, the Earth is far from being a single point, and the sun is really quite big indeed [citation needed]. So we can see the sun for quite some time before the centre point of it rises above the horizon. Likewise, we can see some of it for quite a while after the centre point has set. Even better, the atmosphere bends the sun's light so we see the tip of the sun rising slightly before it is geometrically above the horizon. So even when they're perfectly balanced, we actually get more sunlight than dark! Even better, we get twilight as well. The sun lights up the atmosphere above us long before it rises and long after it sets.
I really like this symbolically: there is a fundamental skew in favour of the light.
Ok, so equinoxes are when the sun crosses the celestial equator. What about solstices? Well, they are the point at which the sun is furthest away from the celestial equator, roughly half-way between the equinoxes.
Again, astronomically, a solstice is both the point in the sky where the sun is furthest from the celestial equator, and the moment in time when the sun is at that point.
The solstices are thus the point in time when the axis of the Earth is tilted furthest (for winter) or nearest (for summer) to the sun.
The winter solstice is also when the sun appears lowest in the sky at its highest point (solar noon). Before the solstice, it's been getting lower and lower, and after, it gets higher. At the precise moment of the solstice, it stands still - hence solstice (from Latin solstitium, sol=sun, stit-= stationary). The summer solstice is when the sun appears highest in the sky, and the sun starts to move lower. The precise moment of the solstice is only solar noon for a single line of longitude on the planet, which is almost certainly not the one you are standing on. But the solar noon nearest to the solstice will be the maximum or minimum you'll see.
The day in which the winter solstice falls is also the shortest, except for some complexity about when you count the start of a day. The times of sunrise and sunset don't quite behave as you would naively expect them to round the solstices either, because of the difference between actual solar time (where the sun is in the sky) and mean solar time (the time we keep on watches). This has another great name: the equation of time. But that's yet another explanation and this post is already too long.
Ok, so an equinox is when the sun crosses the celestial equator, and a solstice is when it's furthest from the celestial equator. But when is that in calendar terms? Well, this gets a little complicated because, as established in the Digressions above, days and years don't line up perfectly. On average the fun and games with leap years keeps the solstices and equinoxes very close to the right points in the calendar, but it isn't perfectly right every year. The solstices are about 21 June and 21 December, but they can be a day either way. The only way to be sure is to look it up.
Well, the only way to be really sure is to do the observations and calculations yourself. I did do a load of this, aaaages ago, and got the same answers you get all over the place. So I say you can trust the obvious reliable sources. The winter solstice this year was at 04:48 GMT this morning, 22 December 2015.
Happy solstice! More light!
This entry crossposted to http://doug.dreamwidth.org/309240.html, where there are
comment(s) not shown here.