May 28, 2022


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The afternoon sun isn't always quite in the south.  Here is an explanation

The afternoon sun isn’t always quite in the south. Here is an explanation

Perhaps your intuition will tell you that it is better at noon to look for the sun exactly in the afternoon. However, this is not always the case. Sometimes the noon sun is on the eastern side of the sky, and sometimes it’s actually on the western side. How it works? lets see

First, let us justify the possibility of observing the sun at noon in a direction other than the south, which results from the sphericality of the Earth and the inclination of its axis of rotation relative to the plane in which the sun rotates.

At noon, the sun can be seen almost towards? Well, in what

Well, while in Poland the sun is already visible even in summer in the direction of “warm countries”, that is, to the south, and the further south, the higher the sun is above the horizon during the day.

Over the Tropic of Cancer, which passes through Mexico, North Africa and India, as well as southern China, the sun will appear directly in the sky on the day of the summer solstice. In turn, the Tropic of Capricorn, which passes through South America, South Africa, and Australia, such a situation will occur at the winter solstice.

Hover is the moment when the orb is above the horizon on a given day. For the sun, we equate this moment, unfortunately, with noon

South of the Tropic of Capricorn, the sun is always visible on the north side. On the other hand, it can be seen on the equator for half the year towards the south, and the other half towards the north. There, the sun rises and sets completely in a vertical direction throughout the year. Therefore, the sunset near the equator is much faster than in Poland, and the difference between the longest and shortest day of the year is small.

Orbits of the Arctic Circle
Tropical region, marked in red, where the sun rises once from the north and once from the south of the sky (Photo: Genetics4Good CC BY-SA 3.0)

In the case of regions beyond the Arctic Circle, when the sun hangs over the horizon for days, it clearly does not change altitude. In practice, every day “at noon” towers at a slightly different height.

Back, however, to that 12:00 o’clock, which hit our guard. Given the above interpretation, at noon, the sun should theoretically be either exactly south or north, perhaps just over our heads. However, this is only the case on certain days of the year or certain places.

What our watch actually shows, i.e. standard time and time zones

The explanation for the first part of this puzzle lies in the word watch. Exactly, what does it measure? What we use the most today is mostly a smartphone or smartwatch synchronized with very accurate atomic clocks. Unfortunately, I must disappoint you, the clock measures official time, that is, the time administratively imposed in a particular area.

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Official time is divided into time zones, the scope of which is usually defined so as not to hinder the work of people and the economy. This is why most of Europe is covered by one time zone, but in the United States, for example, we actually have several of them

We have Central European Time (CSE) in almost all of Europe. So if it is 12:00 in our country, it will also be 12:00 in mainland Spain. However, the sun cannot be there at the same time in the same location in the sky. You just can’t. Solar time, that is, the time caused by the Earth’s rotation, varies at each meridian. In theory, the time will be different on each side of the desk you’re sitting in.

The official time of a particular time zone corresponds to the solar time of the meridian whose angular distance from the primary meridian is a multiple of 15 degrees and the number of the zone

time unit
World time zones. As you can see, there is no division rule here. The main part of the United States, while extending to China, is dedicated unlike the latter

CSE time is actually solar time, and is valid at 15° East longitude. Warsaw, for example, is on the 21° meridian (east of this meridian), so the sun should be here at noon already in the west. Usually, about ten minutes or so after 11:00, the farthest south appears on a given day and the highest in the sky. On the other hand, in Barcelona, ​​which is located near the main meridian, when the sun rises in the sky, the hours appear already at 13:00. Watch noon still on the east side of the sky.

This is why people in Spain go to sleep late, simply because the days are longer than in Central Europe. It’s just a matter of an administratively acceptable hour. The whole world is divided into 24 time zones, but due to the fact that not every country is as rectangular as Wyoming in the USA, time zones do not always correspond to our intuition.

An example of the position of the Sun in the sky on different days of the year at the same official time (taking into account the change to winter time) at the latitude of Poland. The curve formed by these elements is called the analemma. On Earth, it is shaped like an eight, while on Mars it is shaped like a pear (Image: jailbird CC BY-SA 2.0 de)

Night note buffs know very well the above. Clock time is not as important as sidereal time, which determines the positions of stars and other objects in the sky.

Analyma Scheme
Analma chart for the position of the Sun at noon, for locations in the northern hemisphere of the Earth with a latitude of 51 degrees, such as Wroclaw and Lublin, but also with a good approximation of the whole of Poland. Here we have the altitude above the horizon and the azimuth (180 degrees is the direction south)

If the official time is given for each longitude, then this is the solar time, but mean

Well, we know that in order for the time of the clock to correspond closely to the solar time, that is, related to the daily movement and the position of the Sun in the sky, the clock must show the time corresponding to this meridian at every meridian on Earth. This is not technically possible, so the difference in solar time between the eastern and western borders of Poland is several tens of minutes.

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Armed with this knowledge, we go out into the yard, introduce the hourly time correction associated with our distance from the longitude 15 ° (the inhabitants of western Poland are lucky, because this is where the meridian goes), we look at exactly 12 to the south and … this. The probability of the sun rising at exactly noon is about 4 to 365. In any case, you should not look at the sun itself, you only need to stick and measure the direction of its shadow. When you face exactly north, it means that the sun is exactly south.

Average solar time and true solar time. The difference is important

Why are the odds 4 in 365, or roughly one percent? Well, we forgot that the Earth revolves around the Sun in the form of an ellipse, not a circle. in another meaning. We know him well, but he still needs to be taken into account in our calculations. Moreover, it is also necessary to add the fact that the axis of rotation of the Earth is tilted with respect to the ecliptic.

time equation
Time equation, graph of values ​​over the course of the year. When the value is negative, our clocks are ahead of true solar time, while positive is the opposite. If we make a graph of the differences in the position of the Sun at the same time, we get something similar (Photo: Pedro Sánchez CC BY-SA 3.0)

The solar time that corresponds to the equator that we associate with Standard Time and the fact that the Earth makes one revolution in 24 hours is actually the mean solar time. So it would be a real solar time in a circular orbit.

The real solar time can be calculated from the mean solar time or vice versa using the so-called time equation

This discrepancy, that is, the faster movement of the Earth around the Sun in the Polish winter, is slower in the summer, and the effect of the changing tilt of the ecliptic towards the Earth’s equator, is corrected by the so-called equation of time. Its graph is as shown in the graph above. The mean solar time correction is sometimes positive, sometimes negative. Only on April 15, June 13, September 1 and December 25, the correction is zero.

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We perform the actions, we calculate the official time correction to the mean solar and the true sun and we know when the sun will be visible in the sky towards the south on a particular day. Are you sure? Not necessarily, although this error can be forgotten.

The last thing remains, i.e. correctly selected direction to the south

There are other important details left, however we try to be as accurate as possible. It’s not even about the weather, but how we determine which direction to go south.

The compass today points north with much greater accuracy than it did in the nineteenth or twentieth century. This is the result of the migration of the magnetic pole, which is now close to the geographical pole

If you do this with a compass, it will be the direction caused by the position of the magnetic poles, we mean the south direction caused by the rotation of the Earth. How is it set? Preferably at night when the Polar Star helps us. It is she who gives us this opportunity with a very simple mistake. Points north and south exactly on the other side. Residents of the Southern Hemisphere are not so lucky, there is no particularly bright star near the south celestial pole.

Sundials, i.e. those that are neither accelerated nor delayed

The natural clock that shows the true solar time is the sundial. We can find it both in historical places and in a more modern version in gardens. However, even this clock, although it accurately showed true solar time (the error was due to an improper reading of the exact time), had to be built and properly adapted to the place where it was used. Hence the different looking watches that we will meet in different parts of the world. The design of some of them was quite complex, such as the Orloj clock in Prague (photo below). There were also portable solar powered watches.


They were used long after humans gained the ability to make mechanical watches, and the timing system began to take into account time zones. Only in the twentieth century did sundials become mere structures representing historical and cultural heritage. And we? What would we do without the ability to measure time, such as the time I devoted to reading this text? And although we don’t need the Sun to count it anymore, we still take into account its position, and make slight corrections from time to time, called leap seconds. Among other things, they correct irregularities in the Earth’s rotation.