Sun, Moon and Stars
12/Nov/2011 11:05 Filed in: Life in
Timbuktu
Timbuktu is situated between 16° 44’555” and 16° 47’ degrees North latitude and 2° 59’710” and 3° 00’ 977” degrees west longitude. This puts it in the same time zone as England. It is in the northern hemisphere so it follows the general pattern of shorter cooler days in the months of Dec Jan Feb and longer hotter days in the “summer” months of June July Aug. However being only 16 degrees off the equator the seasonal variation in day length and temperature is minimal.
Timbuktu is not on the equator but it is not very far away either so the days do not change by much. In the summer dawn comes at 5:38 am and dusk at 18:49 in the winter the sun rises at the latest at 6:44 am and sets at at the earliest at 17:36. This latest dawn and earliest dusk do not happen on the same day so in fact the length of the day is never shorter than 11 hours 8 minutes or longer than 13 hours 8 minutes. The sunsets can be brilliant and spectacular but are short lived with little twilight separating dusk and night.
For some explanation of reason for these effects see the tutorial below.
The Stars
Being in the northern hemisphere the same stars and constellation are visible as those seen in Europe and North America. However being closer to the equator one might notice that the position of these constellations in the sky is not quite the same. The Pole star will appear lower on the north horizon than at higher latitudes and some more stars will be visible on the southern horizon that are not visible farther north. Some of the constellations only seasonally visible in the northern horizon (they remain below the horizon during parts of the year to due earths axial tilt) are visible for more of the year. For people used to a southern sky there will be a whole new set of stars to view. For those in the north it is a change to rediscover old friends forgotten behind light pollution and smog in the great cities of the north.
The Moon
How the moon looks in the sky depends on where you are on earth. Many people who live in cities with lots of brights lights will not even be able to see the stars and pay little attention to the moon. In timbuktu the cities lights are minimal and the stars extraordinary. When the moon is full or mostly so the light reflecting off the pale sand is enough to easily see by. Even when there is no moon in the sky the stars can provide enough considerable light to the adapted eye. People live their lives to the rhythm of the moon. While they follow the western calendar of months and dates in secular life they use the moon to determine the dates of the religious observances which are based on the islamic calendar of 12 lunar months.
The lunar month begins for the islamic count not on the new moon (where there is no moon visible in the sky) but on the first sliver of the crescent after the new moon. The evening that crescent is viewed is counted as day one of the month. Since the exact length of time before the first sliver of moon is visible varies it is impossible to know in advance on just what day a given month will start. Of course now astronomers have perfected the mathematical formulas which will accurately predict this but many muslims still prefers to rely on eye witness viewing of the moon before confirming the date.
How do you recognize where in the lunar month you are if you have not been following it from the start? In the northern hemisphere, a waxing (growing larger in the first half of the month) crescent will be a right crescent while an waning (getting smaller in the last half of the month) crescent will be a left crescent. In the southern hemisphere a waxing crescent will be a left crescent while a waning crescent will be a right crescent. In the northern hemisphere the Moon moves clockwise across the sky, while in the southern hemisphere it moves counterclockwise across the sky. So what happens near the equator – such as is the case in Timbuktu? The Moon will move in a straight line when seen from above, or straight up and down along an east to west line when seen from the ground. So both the waning and the waxing crescent will be a bottom crescent! In order to tell them apart, you have to think about whether the Moon is in the east or the west and what time it is. Notice that the crescent faces the Sun, so there can never be a top crescent. This only applies to crescents near the horizon. If the crescent is higher in the sky, there is no natural way to define left or right.
You may also notice that the moon comes up later as the month passes. The moon takes just over 27 days to make its circuit around the earth but from our perspective on the earth, which is also moving, it takes about 29 and a half days for the moon to reappear in the same position, that is to go the full 360 degrees of a circle. This averages out to 12.2 degrees a day. It is this angular change which causes the moonrise and set times to grow later each day so that moon rise of the new moon corresponds roughly with dawn and sets roughly at dusk. While the full moon rises around dusk and sets with the dawn. For reasons I do not fully understand instead of this change being an even 48.8 minutes later each day the amount of retard varies. Around the new moon there is about a 70 minute retard in moonrise and only a 30min later set time. As the month progress the delay in rise time shortens and the delay in set time lengthens to around 48 minutes for each and at full moon the phenomenon has reversed with moon rise only 30 minutes later and moon set 70 minutes later. By the third quarter the rise/set times approach, match and cross each other again returning to the original times by the new moon.
What about an eclipse? A solar Eclipse occurs when the moon is truly in complete alinement between the sun and the earth it casts its shadow over the sun gradually darkening out a crescent, then half then for a few minutes the entire disk of the sun is blocked showing only a bright corona around the outside. A Lunar eclipse also occurs when the alignment of sun earth and moon is straight but in this case the earth is between the sun and moon casting its shadow over the full moon. In this case when the shadow fully covers the moon the moon is not blacked out but remains visible as a brownish grey phantom of itself. Note a solar eclipse can only happen during a new moon and a lunar eclipse only during a full moon.
Here is a brief tutorial about why geographic position affects day length and seasons: It is common knowledge that the earth spins on an axis giving rise to night and day as the area facing the sun constantly shifts over a 24 hour period. It is also common knowledge that while spinning around its own axis each day the earth also rotates around the sun each year. We often say that it circles the sun but in fact the orbit is elliptical, it makes a sort of oval about the sun, with the sun not quite in the center of this orbit but slightly closer to one end. This is due to gravity. The much stronger gravity of the sun pulls the earth towards it. As it approaches it moves faster and faster and because of laws of physics that cause you to fly through the windshield of a car that crashes, or keeps water in a bucket that you swing rapidly in a circle, instead of falling into the sun it flies past until the force of the sun pulling it back slows the speed and gradually it reverses direction swinging back around to take another pass on the other side of the sun. When the earth is closest to the sun in one side it is the autumnal equinox and on the other side the spring equinox. Mid summer and mid winter happen at the long ends of the ellipse. This orbit does not actually have much effect on climate. In fact winter (in the northern hemisphere) is at the end of the ellipse that is slightly closer to the sun while summer is at the farthest end of its orbit.
The main factor is that the earth's axis does not go straight up and down but rather it is tilted at an angle (just as you see it on globes). This means at some times the top of the earth (the northern hemisphere) is pointed towards the sun and the bottom (southern hemisphere) is tilted away and at other times the reverse is true. Thus there is summer in the northern hemisphere when the earth is on the part of its orbit wherein the northern hemisphere is tilted more towards the sun which causes the sun to rise higher in the sky, stay above the horizon for longer and the sun's rays to hit the ground more directly. When the Northern hemisphere is oriented away from the sun, the sun rises lower in the sky, its rays are less direct and it remains above the horizon for a shorter period.
Around midsummer (for the northern hemisphere - in June) the north pole is tilted 23.5 degrees towards the sun which keeps all places above 66.5 degrees north in 24 hrs of sunlight while locations south of 66.5 degrees S are in darkness. At mid-winter (for the northern hemisphere - in January) the opposite is true and the north pole is in darkness while the south pole receives 24hrs of daylight. The sun does move through the sky, on a yearly cycle, though, and this is why there is light at the north pole for 6 months and night for 6 months. Thus, above the arctic circle (or below the antarctic circle), at the right time of the year, you can see the midnight sun-- a sun which doesn't set all day. At that time of year, the sun is a circumpolar object, like one of those star that never descends below the horizon.The farther north you are the greater the shift towards or away from the sun and the greater a variation in day length. If you are on the equator you are always the same distance away from the sun and the day's length does not change at all.
Why do we have shorter twilight nearer the equator? The simple answer is that at low latitudes, the sun sets perpendicular to the horizon, while at higher latitudes, the sun can set at a more oblique angle, allowing it to remain close to the horizon after sunset for a longer period of time.
The sun rises and sets because of the Earth's rotation. In the Northern hemisphere each star appears to rotate around the North Star since it coincides more or less with geographic north – like the center post on a merry-go-round. Stars closer to the north star than the horizon never rise nor set, but continually circle the pole star in the sky. Stars farther from the pole star do rise and set, as the circles on which they travel cut below the horizon. The same idea goes for any object in the sky, including the sun and the moon.
Polaris sits directly above the North Pole. If you were standing at the North Pole, you would see Polaris directly overhead. All the objects in the sky would simply turn around that point, and nothing fixed in the sky would rise nor set. If you are standing at the pole, the Earth's rotation simply spins you around in place. You face different directions, but nothing enters or exits the sky above you. The closer to the north pole you are, the higher Polaris is in the sky. At the Equator, Polaris is on the horizon.
If you are far north, but not above the arctic circle. Polaris is very high in the sky, and as the Earth turns, the sun makes a circle that just grazes the horizon. The sun sets, but sets at an angle. It stays close to the horizon for a long time, making twilight linger.
The further south you travel, the lower Polaris will appear in the sky, and the further and quicker the sun will dip below the horizon at night. Eventually, once you get to the tropics, Polaris is on the horizon, and the sun sets directly down below the horizon. Since it has very little movement horizontally with respect to the horizon, it quickly leaves the horizon behind after sunset, making for a very quick twilight. Then, as you proceed southward from the equator, the effect is reversed. Twilights become longer and longer, you see the South Celestial Pole higher and higher in the sky, more objects become circumpolar (i.e. never rising or setting), until eventually you're at the south pole, and the South Celestial Pole is directly over your head, and once again everything is circumpolar, and nothing rises or sets due to Earth's rotation.
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