Due to the inclination of the earth's axis (if we "think away" the earth's rotation around itself for a moment, this phenomenon can be visualized more easily), on the annual elliptical orbit of the earth around the sun, sometimes the northern hemisphere and sometimes the southern hemisphere is closer to the sun. This is how the four seasons are formed in the moderate climate zones.
Since the earth fortunately also rotates around itself (otherwise the sun wouldn't only not rise or set for six months north and south of the polar circles, but it would also not rise or set on an entire side of the globe for six month), if you look at a certain longitude of the earth every day at noon, i.e. at the highest point of the sun, the sun describes a very specific orbit throughout the year - the ecliptic.
For example, if we take a snapshot of the sun on December 22nd each year in Windhoek, Namibia, exactly at noon, when the sun is at its highest, the sun is directly above our heads at a 90 degree angle, the so-called zenith. At all other places along this 15th degree eastern longitude, the sun will not be directly at the zenith on December 22nd, but will only reach less than 90° in height.
Starting from Windhoek situated 15 degrees east on the Tropic of Capricorn, from this winter solstice day onwards (in the northern hemisphere, from whose point of view the name was given, winter prevails on December 22nd), the sun, when it is at its highest at noon during the next three months until March 21st, moving northwards, it stands directly above our heads in the zenith (at a 90 degree angle) at different places: first in Namibia, then in Angola and afterwards in Kinshasa in the Democratic Republic of Congo.
At the spring equinox on March 21, the sun reaches its highest point directly 90 degrees above our heads at noon in Congo at the equator (as the earth rotates eastward around its own axis, i.e. before noon it rotates "towards the sun" and after noon it rotates "away from the sun", we specifically need to watch the time of the highest altitude of the sun on a given day, which is at noon).
At all other places, even within the tropics, where the sun was already or has not yet at the zenith, the sun reaches only less than 90 degrees in height, for example in Windhoek on the Tropic of Capricorn (where we started our example).
But also south of the Tropic of Capricorn in the subtropics and north of the Tropic of Cancer in the subtropics (between 25 to 40° south and north latitude), the moderate climate zone (between 40° and 60° north latitude, e.g. Naples 41° north latitude, Graz 47° north latitude, Prague 50° north latitude, Berlin 52.5° north latitude, Copenhagen/Malmö 55° north latitude or Stockholm 59° north latitude) as well as the cold climate zone (between 60° and 90°, e.g. in the Swedish Östersund 63° northern latitude, at the Arctic Circle in the Norwegian Kaltdalen 69° northern latitude or in Spitsbergen 79° northern latitude), the sun will not reach the zenith, but only a height of less than 90° in altitude all-the-year.
That is not to say that also beyond the tropics and also beyond the polar circles, the sun would not be the one indispensable, light and energy-giving basis of life for flora, fauna and humans alike. What would we do without the sun!
In any case, from March 21 onwards, the zenith of the sun moves further north the equator being highest first at Bocaranga in the Central African Republic (7° northern latitude), then at N'Djamena in Chad (12° northern latitude), the Great Sandy Desert of Bilma in the Nigerian Sahara (15° to 22° northern latitude) until the zenith of the sun reaches the Tropic of Cancer on June 21 in Murzuk at the Libyan-Chad-Nigerian "border" in the Sahara (23,4° northern latitude).
At noon, when I rest a little from the Sahara hike, I can lie there in the sand and can look 90° upwards directly into the sun being in the zenith. Isn't that great! Nobody else can do that along the whole 15th degree eastern longitude from Windhoek to Spitsbergen at this time!
Lying there at this time, the fixed starry sky behind the sun should also contain exactly the border between the zodiac signs of Gemini and Cancer, because on this day of the summer solstice, when the days become shorter again in the northern hemisphere, the sun enters the zodiac sign of Cancer from an astrological point of view.
However, I will not be able to see neither Cancer nor Gemini in the sky, because the scorching hot sun is just too bright.
Assuming that I stay there for exactly twelve hours and assuming that I am still alive, then it is deepest night in Murzuk in the Libyan Chad Nigerian Sahara:
Since noon the sun has slowly "moved away" from the zenith (looking 90° upwards) and westwards to the horizon. With sunset it has already crossed the 0° horizon line.
As the stars rise in the east, the sun "moves" steadily "further" below the horizon. Now - in the deepest night of June 22nd - the sun is directly below me, i.e. I no longer see it looking 90° upwards in the zenith, but looking - 90° downwards in the direction of the so-called nadir.
If I could look down there in the night desert through the sand and look straight through the earth and on the other side of the earth back up from the ocean floor of the Pacific Ocean up to the sea surface and through it, I would see the South Pacific daytime sky at the Cook Islands near New Zealand (165° west longitude, 23.4° south latitude).
There it is currently noon of June 21, since I am still ahead of the dateline 180° west/east longitude, where, if I cross it e.g. towards the Fiji Islands from west to east longitude towards the east, I have to add a whole day (among sailors it is therefore also said: "From east to west, hold the date; from west to east, let go of the date).
However, since I do not "dream" my way out of the Sahara desert through the sand and earth up to the Cook Islands towards the Tropic of Capricorn on December 21, but half a year later (or earlier, depending on the perspective) on June 21, the sun is not directly above me in the zenith at noon of the Cook Island sky, but it only reaches a height of less than 90 degrees.
To be able to see the noon sun 90 degrees above me at the zenith on this day of June 21, I would have to dream myself quickly from the Cook Islands on the Tropic of Capricorn along the 165th degree west longitude northwards to the Tropic of Cancer, and lo and behold: I am in Honululu, Hawaii (165 degrees west longitude, 23.4 degrees north latitude).
That sounds promising. Oh, how nice, first of all relax: "Aloa Hé, aloa hé ... we dance Hula, Hula, and drink a cup of tea ... Aloa Hé, aloa hé ... the sun is shining, time for "artful" rhyming."
Okay, now we can proceed.
Because even if the scorching "Honululu Sun" was not in the air in the zenith at noontime of this 21st June as we dance Hula-Hula and drink coconut milk while listening to the sound of waves and ukulele and me lying in the sun between palm trees in a hammock (because only a minimal shadow beneath the palm trees due to the 90° sun in the zenith, sunlight reaches me in the hammock in between the palm trees), I will not be able to see the zodiacal signs of Gemini and Cancer, whose boundary the sun crosses on this day of the summer solstice when it astrologically enters the zodiacal sign of Cancer, - even here in Honululu.
So no need to bury the head in the sand!
But if, regardless of whether I am in Hawaii or the Sahara (or anywhere else in the world), I do not see the zodiac signs of Gemini and Cancer with the naked eye because of the sunlight on and around June 21st (or if so, then only increasing parts of the zodiac sign Gemini very briefly in the morning before sunrise or decreasing parts of the zodiac sign Cancer very briefly in the evening after sunset), then this also means that I should be able to see the opposite zodiac signs of Sagittarius and Capricorn at the nightly Tropic of Cancer (whether in the Sahara or on Hawaii) very well and for a long time.
Rising on the eastern horizon right after sunset, the zodiac signs Sagittarius and Capricorn are visible to me - lying there in the Sahara sand - the whole night through.
Towards the middle of the night, they ascend to their zenith 90° upwards, whereupon they slowly descend again in the second half of the night westwards, until they finally set on the western horizon in the early morning dawn, when the first sunrays coming from the east flood the sky.
Therefore, not sticking my head into the sand, at midnight, I look up 90° towards the zenith: above me - this is my reasoned anticipation - I would now be able to see exactly the border between the zodiac signs of Sagittarius and Capricorn...
... I try to keep the attention as focused as possible, which is a real challenge...
... the overwhelming beauty of the starry sky rising above the nightly desert dunes of the Sahara is simply incredible...
... whereever I look, there is this twinkling and glittering of (shooting) stars covering the beautiful sky of this chilly desert night. What a magnificent sight!
(continuation in the post "Precession of the earth's axis")