EARTH, EQUATOR AND EQUINOX

Interview with Professor Tom Marsh, Department of Physics

The autumn equinox - which falls this week - is mistakenly thought to be the time of year when day and night are of equal length. In fact, this occurence, known as an equilux, can be days after the equinox itself. Professor Tom Marsh, Head of the Astronomy and Astrophysics Group, argues that more significantly, equinox are a reminder of the fragile state of life on Earth. Life is dependent on particular climatic conditions which could be devastatingly changed if the Earth's axial tilt - which creates the equinox - alters by only a few degrees.

In the northern hemisphere, the autumnal equinox on September 23rd officially marks the beginning of autumn and the progression into winter. About 4 billion years ago a giant impact between Earth and a Mars-sized mass amplified Earth’s obliquity to 23.5°. This was a hugely significant event, also thought to have created our moon. The axial tilt affects everything to do with our climate; a different obliquity may have resulted in no seasons and we may not even have existed as a human race. With no tilt the angle of incidence would have been 0°, perpendicular to the Earth’s orbital plane, possibly causing freezing temperatures and solar radiation to always hit the equator as the Earth rotated. Interestingly, this latter result is in fact what is happening during an equinox; at this time the Earth’s axis is tilted towards the Sun’s rays as the Sun crosses the celestial equator.

Two annual equinoxes occur because of the axial tilt, which consequently produces our four seasons. Viewing the universe heliocentrically, the tilt means that for half of the year (March to September) the northern hemisphere tips toward the Sun during orbit, producing our summer, and away from the Sun for the latter half, leading to winter. The word ‘equinox’ derives from the Latin ‘equal night’ - at this time, day and night should be of equal length. However, this is not the case and depending on our location on Earth, equiluxes can occur days before the vernal equinox, or days after the autumnal equinox. Professor Tom Marsh, Head of the Astronomy and Astrophysics group in the Department of Physics at the University of Warwick, comments that equinoxes are a reminder of the formation of life here on Earth which could not exist without the appropriate climatic conditions.

...a significant change in obliquity could lead to extreme climates. Lower obliquity could result in another ice age...

In addition, Prof Marsh describes the equinox as an example of original, pure Newtonian physics. The direction the Northern Hemisphere points is effected by a phenomenon known as ‘the precession of the equinox’, which was first described by Sir Isaac Newton in book III of Philosophiae Naturalis Prinicipa Mathematica where he applied his laws of motion to astronomy. Each year the equinoctial points shift slightly westward along the ecliptic. “This is a consequence of the ‘wobble’ of the Earth’s axis, which can be likened to the motions of a child’s spinning top as it nears the end of its fast spin. The ‘wobble’ describes an imaginary conical shape in the North sky which now happens to be pointing at the star Polaris, so this is our current North Star. The precession causes a change in North Star approximately every 25,000 years; the time taken for Earth’s axis to precess its full orbit. In Ancient Egypt, for example, the North Star was Thuban. This ‘precession’ occurs due to the gravitational forces exerted by the moon and Sun on the equatorial bulge of the Earth.”

So what would happen if the axial tilt altered? Currently our tilt naturally varies between 21° and 25° degrees over a period of 41,000 years. Research into glacial cycles has shown that this gradual change in tilt effects temperatures by about 4°C and causes the oscillation of glacial formations. This demonstrates how profound the effects would be if our axial tilt was to permanently change, even by a few degrees. Prof Marsh explains that a significant change in obliquity could lead to extreme climates. Lower obliquity could result in another ice age but higher obliquity, scientists believe, would make it extremely difficult for land-based, warm-blooded life to survive as it does today. The three dominant factors that affect the Earth’s climate (axial tilt, precession and eccentricity) are collectively known as the Milankovitch Cycles.

Jupiter and Venus which tilt just 3° share our northern spring, but seasons on these planets are barely noticeable and do not vary greatly between one and the other.

To get an idea of how it could have been if the impact 4 billion years ago had had a different outcome, we can look towards our solar system. Jupiter and Venus which tilt just 3° share our northern spring, but seasons on these planets are barely noticeable and do not vary greatly between one and the other. Mars, Saturn and Neptune have axial tilts and four seasons similar to our Earth, however, other factors, such as distance from the Sun make their seasons much longer and more extreme. Uranus tilts at an angle of almost 90° causing the North pole to point directly at the Sun for half its orbit, leaving the South pole in darkness. This produces long severe seasons. Mars and Saturn’s moon Titan display polar ice cap formation during winter which disappear during summer, similar to Earth. However, due to the planets’ distances from the Sun, all are much colder than Earth and so not directly comparable.

To the laymen, equinoxes may not seem important but the processes behind them are of great significance to physicists. The most noteworthy and perhaps chilling concept is the correlation between the 4°C temperature change due to Earth’s natural varying tilt (and the natural effects this has) and the 4°C changes causing global warming, brought about by mankind.

Prof Marsh leads the Astronomy & Astrophysics group in the Department of Physics at Warwick. His research interests are accretion in, and the evolution of, binary stars. He is also interested in the observational techniques needed to study these objects. His major project of the last few years has been use and exploitation of ULTRACAM, a high-speed CCD camera. He is also now working on L3 CCDs, which offer a potential advance in photon-starved applications such as high-speed spectroscopy. His work touches on the area of gravitational waves as the objects he studies are thought to be amongst the strongest emitters for the proposed space-based GRW detector, LISA.