Joseph Adhemar
Encyclopedia
Joseph Alphonse Adhemar (1797–1862) was a French mathematician
. He was the first to suggest that ice age
s were controlled by astronomical
forces in his 1842 book Revolutions of the Sea.
The Earth's orbit
is elliptical
, with the Sun at one focus; lines drawn through the summer and winter solstice
; and the spring and autumn equinox; intersect with the sun at right angle
s. Currently, the Earth is closest to the Sun (perihelion) near the northern hemisphere
winter solstice, and the earth moves faster through its orbit
when closer to the sun. Hence, the period from the northern hemisphere's autumn equinox to winter and spring is shorter by around seven days than the period from spring to summer to autumn; the reverse is true in the southern hemisphere
. Hence, northern hemisphere winter is shorter.
Because of this, Adhemar reasoned that because the southern hemisphere had more hours of darkness in winter, it must be cooling, and attributed the Antarctic ice sheet
to this. Adhemar knew of the 22,000 year cycle of precession of the equinoxes
, and theorised that the ice ages occurred in this cycle.
One immediate objection to the theory was that the total insolation
during a year does not vary at all during the precessional cycle, only its seasonal distribution. Another was that the timing was wrong; however this could not be tested by the observations available at the time.
Adhemar's theory was further developed and greatly modified, first by James Croll
and later by Milutin Milanković
.
Adhemar theorised about the thickness and predicted the Antarctic ice sheet (the continent had not been sighted) by comparing the depths of the Arctic and circum-Antarctic oceans. Finding the Antarctic oceans deeper (the measurements he used may not have been fully representative) and attributing this to the gravitational attraction of the Antarctic ice sheet, he postulated a truly enormous ice sheet approximately 90 km thick.
Mathematician
A mathematician is a person whose primary area of study is the field of mathematics. Mathematicians are concerned with quantity, structure, space, and change....
. He was the first to suggest that ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
s were controlled by astronomical
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
forces in his 1842 book Revolutions of the Sea.
The Earth's orbit
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...
is elliptical
Ellipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...
, with the Sun at one focus; lines drawn through the summer and winter solstice
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...
; and the spring and autumn equinox; intersect with the sun at right angle
Right angle
In geometry and trigonometry, a right angle is an angle that bisects the angle formed by two halves of a straight line. More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles...
s. Currently, the Earth is closest to the Sun (perihelion) near the northern hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
winter solstice, and the earth moves faster through its orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
when closer to the sun. Hence, the period from the northern hemisphere's autumn equinox to winter and spring is shorter by around seven days than the period from spring to summer to autumn; the reverse is true in the southern hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
. Hence, northern hemisphere winter is shorter.
Because of this, Adhemar reasoned that because the southern hemisphere had more hours of darkness in winter, it must be cooling, and attributed the Antarctic ice sheet
Ice sheet
An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² , thus also known as continental glacier...
to this. Adhemar knew of the 22,000 year cycle of precession of the equinoxes
Precession of the equinoxes
In astronomy, axial precession is a gravity-induced, slow and continuous change in the orientation of an astronomical body's rotational axis. In particular, it refers to the gradual shift in the orientation of Earth's axis of rotation, which, like a wobbling top, traces out a pair of cones joined...
, and theorised that the ice ages occurred in this cycle.
One immediate objection to the theory was that the total insolation
Insolation
Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day...
during a year does not vary at all during the precessional cycle, only its seasonal distribution. Another was that the timing was wrong; however this could not be tested by the observations available at the time.
Adhemar's theory was further developed and greatly modified, first by James Croll
James Croll
James Croll was a 19th century Scottish scientist who developed a theory of climate change based on changes in the Earth's orbit.-Life:...
and later by Milutin Milanković
Milutin Milankovic
Milutin Milanković was a Serbian geophysicist and civil engineer, best known for his theory of ice ages, suggesting a relationship between Earth's long-term climate changes and periodic changes in its orbit, now known as Milankovitch cycles. Milanković gave two fundamental contributions to global...
.
Adhemar theorised about the thickness and predicted the Antarctic ice sheet (the continent had not been sighted) by comparing the depths of the Arctic and circum-Antarctic oceans. Finding the Antarctic oceans deeper (the measurements he used may not have been fully representative) and attributing this to the gravitational attraction of the Antarctic ice sheet, he postulated a truly enormous ice sheet approximately 90 km thick.