2.2: THREE FACTORS’ EFFECTS ON EARTH’S CLIMATE:
2.2.1: TILT OF EARTH’S AXIS:
The tilt of the Earth is responsible for our yearly cycle of seasonal weather changes. Two factors change during the course of a year to give us seasonal variations in temperatures:
I) The angle at which Sunlight enters the Atmosphere and hits the ground:
Fig 12: Angle of Sunlight
Fig 13: Sunlight to Earth
Our atmosphere absorbs and reflects a lot of solar radiation into space. If sunlight enters the atmosphere at a direct angle, as it does in the Northern Hemisphere during the summer, it will go through less of the atmosphere. Sunlight reaching the ground is also more concentrated. Sunlight entering the atmosphere at the Southern hemisphere at this time has to pass through more of the atmosphere and is dispersed over a larger surface area. This situation will be reversed in six months when the Northern hemisphere will be tilted away from the Sun.
Fig 14: Summer and Winter
II) The number of daylight hours:
The Sun appears lower in the sky, rising later and setting sooner, during winter, when the hemisphere affected is tilted away from it. This decreases the number of hours for the Sun to heat the ground and lower temperatures result.
Without the Moon to stabilise the angle of tilt, such seasonal changes would start to vary much more. Decrease the tilt to 0 degrees and no seasons will occur as neither factor above will vary with time. Increase the angle and seasonal variations will become more extreme, with very hot summers and very cold winters as both angle of sunlight entry and number of sunlit hours vary much more between summers and winters. Taking an extreme case like Uranus, whose angle of tilt is around 98 degrees, summers and winters last half a Uranian year each. Unfortunately for us, we do not have a thick atmosphere like Uranus to redistribute heat as efficiently and smaller variations could make most latitudes uninhabitable. Even with the reasonably steady range in which the Earth’s angle of tilt keeps, serious changes such as the Ice Ages have been blamed on it. The Sahara Desert was once a lush jungle until a one- degree change in the axial tilt turned it into the dry, burning wasteland it is today. A fifty-degree tilt would send the Sun overhead at the poles and melt the Ice caps. Resultant changes in sea level would submerge much coastal land.
Seasonal winds such as the Monsoons are dependant upon differential seasonal heating of the Asian interior, and the different rates at which land and sea absorb and lose heat, to generate them. The intensity of the winds depends on the temperature gradients, and heating of the continental interior in the summer, and cooling in the winter, create steep ones, which lead to the strong winds. This gives much of Asia its rainy season. Extreme variations in the heating and cooling, due to huge variations in the Earth’s rotational axis, would either degrade the temperature gradient or intensify it. With such unpredictable seasons, rice growing might never have taken off successfully on a large scale. Natural ecosystems would also suffer from any wild variations and extremes.
2.2.2: SPEED OF AXIAL ROTATION AND THE CLIMATE:
Doubtlessly, the wind speeds on a moonless Earth would have led to a much harsher environment. Taking Jupiter, which has a high speed of rotation around its tropics, as an example, the Giant Red Eye on the planet is actually a long-lived violent storm, seen first by astronomers three hundred years ago!
Fig 15: Hubble views ancient storms
in atmosphere of Jupiter
Fig 16: Ground hugging tree. Example of how life might adapt to constant strong winds.
As a result of the rotation of the Earth, large storm systems form around low-pressure centres. A greater speed of rotation and greater amounts of hot air available in extreme summers, would naturally lead to more frequent and violent hurricanes, typhoons and cyclones. (Hot air is needed to fuel instability and hurricanes tend to move towards hotter areas at higher latitudes). Life would have evolved very differently under such conditions. Because of high rates of erosion and weathering, land-formation might not keep pace. This could eventually turn the planet into a water world, where no fire-discovering species would have evolved to create tools, machines, civilisation and astronomy!
Precession does not generate climatic change without a tilt in the axis of rotation. What it does is to change the orientation but not the angle of the earth’s rotational axis. This causes the seasons to migrate very slowly in a 26,000-year cycle in relation to our solar calendar. The climate itself is not affected but the timing of the seasons, in terms of which months they start in, does. In 13,000 years, the winter solstice would occur in June!
2.2.4: EFFECTS ON TIME KEEPING:
The 23.5-degrees tilt in the Earth’s rotational axis gives us the ecliptic (the apparent path of the Sun in a year) and the resultant Analemma. There would be no changes in sunrise and sunset times and length of days without it. Calendar making and time keeping evolves slowly as we unlock such complications, developing a knack for mathematics along the way. Relative stability is therefore a very important pre-condition for this process. Without the Moon to steady the tilt, there would be large and more frequent variations to these patterns. Would ancient astronomers have been able to keep up? If data becomes obsolete too soon and relatively accurate predictions of the movement of the Sun become impossible, would these ever be used as a basis to measure time? With no moon to act as an easy alternative, would anybody have had any idea on how to make a useful calendar? Even length of days would vary rather unpredictably.
Would precession become less pronounced without the Moon? There are no studies on this subject to be found. Complications like the Earth’s own internal dynamics and the gravity of other planets might become important if the Moon was removed from the picture. Precession affects time keeping by changing the position of the Celestial Poles, thus moving seasons and their markers (solstices and equinoxes) with respect to the tropical year calendar we now use. Variations in the pattern of orientation could change calendars from time to time. However, unless these changes become very frequent, and there is no concrete evidence to show that it will, mankind should be able to cope with such minor inconveniences
Next (EFFECT OF THE MOON ON TIDES)