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Astronomical question and answer 254


Frank L. Preuss


When does the sun set at the same time, when the moon rises?


At full moon.

Today is full moon and the sun is already preparing itself for the setting.

I have looked up the times for sunset and also for moonrise. Moonrise is a quarter of an hour after sunset.

Those are therefore two natural spectacles taking place at the same time.

I therefore sit here at my computer and can see the sun from this place. I have just looked at the sun and the result was that I have now problems seeing the screen.

The sky is relatively free from clouds. It is a little hazy and there will probably be a beautiful sunset.

My view upon the moonrise is blocked through a residential block and I expect to see the moon to rise so about two hours after moonrise above the block.

When one busies oneself with these data it becomes easier predicting positions of celestial bodies.

I have now looked out of the window, which is towards the east. Above the block the view is not different than towards the west, partly overcast.

The interesting thing today is that it is only 4 days until solstice. Full moon is therefore close to the point of time, where the sun on its way along the horizon, there where it rises in the morning, and where it sets in the evening, moves very slowly. And that means that the times of its rising and setting hardly differ from day to day. And the horizontal angle to the north direction practically does not change at all.

From the experience of the last years I know, where the sun will set. It is that point of the sunset at the horizon, which deviates most from the west direction.

When I measure the vertical angle, which the course of the sun will take until the setting, then the angle corresponds to the angle of my parallel.

I have just now measured the angle. That is not so easy, because one has to look into the sun. But the sun gets the weaker the more it moves towards the horizon.

When I know, where the sun will set, I can measure this angle directly; one does not need to plot the shadow somewhere.

I have missed to observe the moon setting, the full moon setting, this morning, and to then to go from my window towards the west to my door in the east, and to then, 10 minutes later, experience the sun rising.

I have only experience the sun rising; I did not think of the moon setting. Such a relatively rare natural event, as to experience a moon setting and sun rising at the same time, I therefore missed. I probably had to be more aware that it will take place, to then observe quite purposefully. I also was lucky insofar that the sky was clear in the west as in the east. And also, that from there, where I live, can see the western as also the eastern horizon. And then also the time of the year has to be added that I can only see the horizon in the east, where the sun rises, in this time of the year.

There was a thin layer of clouds above the horizon and I expected to see the sun only then, when it appears above these clouds. But then I still saw it rise directly above the sea, but veiled. So those were no clouds, but a hazy layer.

The sunrise I can see, but not the moonrise. There is about 45 degrees difference. At new moon this difference is zero degrees. There one can therefore see, how within two weeks, the course of the moon deviates from that of the sun.

I have just now looked at the sun set. The sky is almost cloudless. The sun set exactly there, where I had expected it.

It is now half an hour after sunset and the horizon is blood-red in the area of the spot, where the sun set.

It is now four hours after sunset and after moon rising and I was before the door and the full moon stands high up in sky in the east, far above the building, and also there, where I expected it.

I have now again been outside. The sky seems to be cloudless. I saw two stars and in between the full moon, almost directly above me. The time is after midnight.

It is now half an hour before the sun is rising and a good hour before moon setting. The morning twilight has started in the east and in the west the moon still stands relatively high in the sky. The sky seems to be cloudless.

The sky in the west is still as good as completely black, but now starts to turn into blue, and I expect the change of the moon from a moon, which one sees at night, and one which one sees during the day.

But I therefore still have the opportunity to experience a sunrise and a moonrise at the same time.

The moon is a radiating yellow disc in the sky. The edge of the moon is clearly visible and also the moon spots. It is now one day after full moon, the waning moon, but I cannot see anything of a waning.

The rosy dawn is now visible, particularly at some hazy places, also in the west. The horizon in the east has a layer of clouds, but it is probably again just haziness.

It is now five minutes before sunrise, and I now want to see, whether the time, which the astronomical table gives for the clock time of sunrise, agrees with the real sunrise.

There is no colour difference between the sea and the sky; but I know where the horizon is, from buildings, which I take as marking points.

The sun has now risen, but three minutes after the given time and also a little higher than the horizon. So they were still clouds, and no hazy layer.

The sun now stands fully above the horizon. It is an impressing picture.

The moon has suddenly changed from a yellow colour to a white-grey. Only just a little yellow is left. The sky is already completely blue.

Time passes so quickly that one does not have enough of it to be aware of the details of the changes.

The moon is now still well visible, but the more it is going to the horizon the hazier it gets.

It is now 10 minutes after the theoretical sunrise.

The sky is now blue and cloudless, but there, where the moon is, well above the horizon, it is just grey; but the moon stands out against it with its white-blue.

It is now half an hour before moonset.

The moon now takes on more and more the same grey, which its surroundings have.

The sun is now the centre of a gigantic garland of rays and one must now already search for the moon.

It is now 20 minutes before moonset and I doubt that I will be aware of the moonset, and can compare the time of the setting with the table. The moon is now only visible for someone, who pretty well knows, where he can find it.

I have now again looked at the moon, and did intentionally not look into the sun before, so that I do not get blinded, but the moon will probably no longer be visible in the next few minutes.

It is now 11 minutes before the theoretical moonset, but the moon is no longer visible; it is too hazy.

The horizontal angle, which the astronomical table gives for the place of the moonset, agrees so approximately with the estimated angle. The vertical angle of the course of the moon in the sky I did not measure, because I did not know the place of the moonset.

I have now measured the vertical course of the sun orbit, what was well feasible, since I can still look into the sun and also know the place at the horizon, where the sun has risen, and this angle agrees with the angle of my parallel.

Today are three days before solstice and consequently the place of sunrise is to fairly exactly the place at the horizon, which is furthest away from that place, where the sun rises at equinox, east. The astronomical table even shows that today already falls into that area of days, where the deviation has a maximum; and that for sunrise and sunset.

For moonset and for moonrise it is even today the day, on which the moon deviates most from the course of the sun, where the horizontal angles of the setting and of the rising at the horizon show the greatest deviation from the course of the sun. At new moon the points of the setting and of the rising agree with those of the sun.


This is the end of "Astronomical question and answer 254"
To the German version of this chapter: Astronomische Frage und Antwort 254



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