Caoimhín's Content

I mentioned in the last article that there are different ways to measure a day, among them are the sidereal and solar days. This little tidbit was not mentioned in the previous video attached to that article, just one of the many little bonus facts readers of the website get. In the above video, I go through the difference in a little bit more detail. I will begin with the most familiar definition, the solar day. This is the day that we normally use for our earthly, the standard 24 hour day.

The solar day is, unsurprisingly based on the Sun. This is the length of time it takes for the Sun to return to the same location in the sky. Thanks to the tilt of the Earths orbit, this is easiest to measure at midday. It it is best to measure it at local or physical midday, the moment when the Sun is directly above the South. Right now, for us here in Ireland at least, the clock will say that it is roughly 1:30 when we reach our physical midday. This is because we are after changing the clocks by an hour. Ireland is currently using daylight savings time, so the Sun will get above the South an hour later we might normally expect. Even without daylight savings, the Sun won’t reach directly above the South until a little after 12 o’clock here in Ireland. We are in a time zone centered on Greenwich in London, this Greenwich Mean Time was the world standard, now called Universal Coordinated Time. This means that our physical midday is a little delayed because we say it is midday here, when it is really midday in Greenwich. This is only a small delay, but the hour added by daylight savings extends the difference even more. With the Sun directly above the South, if we move through a solar days, the Sun’s position will change slightly. Firstly, it will get a little higher in the sky, as we get closer to the summer solstice. It also appears to move, very lightly, from side to side. If you go back and take a look at the article and video describing the analemma, I explain how and why the Sun moves in a figure of eight or lemniscate shape over the course of the year. It’s a tough phenomenon to notice, as the movements are rather subtle, but with careful measurements you can see that the Sun ends up running late as we come to Midwinter and then runs a little fast as we move past it. The analemma does slightly change the position of the Sun, but only by a few minutes. This is partly due to our motion around the Sun, and this is also what causes the two different kinds of day.

Starting from a fixed location on the Earth, with the Sun at it’s highest in the South, if we measure how long it takes for the Sun to come back to that position, it will be almost exactly 24 hours. We often measure the day from one midnight to midnight, or from one sunrise to the next, but in astronomy it is common to measure them from noon to noon. This set of 24 hours is a solar day. At this point in the video, we are looking at the daytime sky with no atmosphere, so the stars and planets can be seen around the Sun. Venus is behind the Sun, but the others are out to the side, Saturn, Mars, Mercury, Uranus, Jupiter, and even Neptune, as well the comet 12P/Pons-Brooks. Moving through solar days in a view like this, the planets will move as they orbit around the Sun and the Sun appears to stay in pretty much the same place. The background of stars will appear to slide past behind the Sun, any given star getting further West each day.

While we turn around to face the Sun, we also move around the Sun, every day we’ve moved a little bit further in our orbit. This means we need to turn a little bit further for the same part of the Earth to be facing back towards the Sun directly. Instead of measuring how long it takes for the Sun to return to the same spot, we could instead pick a distant star, and use that as a marker instead. Aldebaran is a good example at this time of the year, if you don’t have to worry about the atmosphere. Aldebaran, the brightest star in the constellation Taurus is almost in line with the Sun on the 31st of May, visible just a little below it in an atmosphere free view. This view also lets us see Orion, which is currently in the same part of the sky as the Sun, preventing us from seeing it in the evening. With Aldebaran and the Sun in the center of our sky, if we move through sidereal days, the Sun will appear to drift East, while the stars remain fixed. Truth is that the stars do stay in the same place, even the Sun remains relatively fixed over human timescales. However, we are moving around the Sun, we’re not moving around these other stars. In the same way that a solar day is measured by waiting for the Sun to return to the same part of the sky, a sidereal day is measured by how long it takes us to turn around to face a more distant point. We can essentially treat the stars as fixed points in the sky given that they are so so far away. The stars may appear to move a little over short timescales, if their position is measured very accurately and precisely As we orbit the Sun, we travel, 2 AU, from one side of the Sun to the other side of the Sun. As we are one AU away from the Sun, an AU is the average distance from the Earth to the Sun, from one side of our orbit to the other is 2 AU, 1 AU to the Sun and another to the opposite point in our orbit. This can cause a little bit of wobbling with some stars, a phenomenon known as parallax. We can use this to measure how far away different objects are, but it isn’t really an issue when you are only moving a day ahead in the Earths orbit.

It takes slightly less than 24 hours, about 3 minutes less, to turn around until the same part of the Earth is facing a distant star in the sky, and because the Earth has moved around in its orbit, we then have to turn a little bit further before the Sun comes back to the same place it was the day before. This means that the solar day is a little bit longer than the sidereal day, because the solar day isn’t just about the Earth turning around, it’s about the Earth turning around while it’s orbiting the Sun. The actual length of time it takes for the Earth to just rotate 360° is closer to the sidereal day then the solar day because we have to turn just a little bit more than 360° to realign ourselves with the Sun, about 361°. That’s the difference between a solar day and a sidereal day, 1 degree of our rotation, about 3 minutes.

I brought up the sidereal day in the last article due to the Plough or Big Dipper. This famous shape in the sky will appear at sunset in slightly different positions at different times of the year, right now it appears practically at the zenith of the sky once it becomes truly dark. Facing towards the North Star, if we move through solar days it will appear that the Plough is moving around the North Star, if we moved through sidereal days, they don’t really seem to move it all. If we move through hours or minutes instead of either kind of day, our rotation will cause the stars to appear to move. That is due to our rotation alone, but the change in position from season to season are due to our use of a solar day, the whole point of a sidereal day is that the stars are in the same relative position, so measuring like this would mean the constellations would appear in the same place after every sidereal day. Unfortunately, they would get blocked out by the Sun, thanks to its position following the solar day instead.

Moving ahead a few days into early June, we can still see all those planets if the atmosphere is removed. No atmosphere gives you a perfect flawless view of the Milky Way, if you can get up above the atmosphere, which is much easier said than done. If you’re an astronaut then you’ll have that unrivalled view, if you are a space tourist it really depends how far up your rocket goes. To get a truly perfect view, you do need to be outside the atmosphere. Neptune is not visible to the naked aye, but it is above the horizon, a little bit closer to the position of Saturn than Mars, a reasonable distance from the Sun. This is purely based on the apparent positions of the planets from the Earth, ignoring their actual relative positions in space. Neptune is really hundreds of thousands of kilometers away from either of Saturn or Mars, who are comparatively closer together, even with the whole planet Jupiter’s orbit occupying the intervening space. Uranus as I’ve mentioned many times, is not quite visible to the naked eye. With no atmosphere at all, you might just be able to see it, but even in a pristine dark sky here on Earth, it’s going to be a little tougher to see.

Living under the atmosphere as we do, the light of the Sun is scattered in our atmosphere, lighting up the sky and blocking the planets from view, during the day at least. Being in a dark sky does help, as the light of sunrise alone is still a little less than sunrise plus light pollution. Early in the morning in good conditions, we can certainly see Mars and Saturn from here in Ireland, no one will get to see Neptune or Uranus, but we are also missing Jupiter and Mercury. They are all above the horizon before the Sun rises, but we only get to see two of them here in Ireland. Venus as I mentioned is right on the far side of the Sun. If you were closer to the equator you might have a better chance to see this Parade of Planets as people are calling it, 6 planets in a row, with 4 technically visible to the naked eye. Jupiter and Mercury would look very nice on the 4th of June having a close conjunction, but it’s not possible to see from here in Ireland, or any of the more northern latitudes.

To get a better view, we’ll move the viewing location to the United States. I went with about halfway down the East Coast of the United States. In the video, I erroneously call it the West coast, it is far to the West of me after all. One of the nearby locations to the dropped pin was Annandale. Despite my patchy knowledge of the USA or the Americas more broadly, I had a feeling that this was close to Washington DC. Upon further research, it looks like Annandale is inside the city of Washington, or at least inside the conurbation it has stretched into, even if it is technically in the state of Virginia. From this lower latitude, it becomes possible to see both Mercury and Jupiter, very close together, just above the Sun at sunrise. It is mostly Jupiter rather than Mercury that you can really see, despite Mercury being practically in front of Jupiter, Jupiter is significantly brighter. I do think it is important to emphasise how difficult it actually is to see these 4 planets in a row. The Sun needs to be very close to rising for Jupiter and Mercury to be above the horizon, and even then they don’t get very high before the glow of the Sun washes them out. This latitude is comparable to being around the Mediterranean here in Europe, so it will be impossible for us here in Ireland. If you are in another part of the world, you might just get to see four planets in a row, but it will be tricky and no one gets to see Neptune and Uranus without a telescope, so six will only be possible if you have that telescope to help you.

Despite the difficulty, you might get to see four planets together in the sky at least, so I hope that if you are in another part of the world you will get to see the. regardless of the Parade of Planets, I hope you enjoyed my description of the difference between a sidereal and a solar day, if you did then I hope you subscribe to my website, YouTube channel or both, and I hope that you will return for the next post.