The solstice and perihelion and how they relate to summer and the calendar

Earth path

Sketch diagram showing the path of the Earth around the Sun. The path appears as an oval not because of the slight difference of the path from a circle, but because of the oblique angle of view. Diagram Nick Lomb

A recent breakfast radio discussion (hello Red S) suggests that the effect of perihelion on temperature and the seasons is not clear to many people. Admittedly, much of the supposed basics about the path of the Earth around the Sun are in reality fairly complex. So here is a brief primer.

The seasons are mainly due to the 23.5° tilt of the axis of the Earth as it moves around the Sun. It is summer in the southern hemisphere when that half of the Earth is tilted towards the Sun. Six months later the Earth is on the other side of the Sun and so it is summer in the northern hemisphere as that is then tilted towards the Sun.

The day when the tilt towards the Sun is at its maximum is the summer solstice. On that day the Sun is at its highest in the sky and is above the horizon for the longest time in the year. Hence the day is also known as the longest day. In the southern hemisphere that day is around 21 December each year.

Our calendar is based on the seasons which are governed by the tropical year – this is the interval from one solstice to the next, say summer solstice to summer solstice. It is approximately 365.2422 days. The length of the calendar year tries to match this while having whole number of days in each year using a system of leap years. The current Gregorian Calendar yields a year of 365.2425 days, which is a good match to the tropical year.

Earth distance from Sun 2013

The changing distance of the Earth from the Sun during 2013. Diagram Nick Lomb

The path of the Earth around the Sun is not a perfect circle, but is slightly oval-shaped. In early January the Earth is closest to the Sun at a time known as perihelion, while in early July it is at its furthest at a time known as aphelion. The variation in distance between perihelion and aphelion is approximately 3%.

As perihelion occurs at the height of the Australian summer that could suggest to some people that it causes the seasons. It does not as is obvious by the consideration that the northern hemisphere summer occurs at aphelion. However, the perihelion in January does make the southern summer a little hotter than otherwise and, more, importantly shorter than otherwise as the closer the Earth is to the Sun the faster it is moving.

Relief is at hand though as perihelion will not always occur in January. There are year to year fluctuations, but on average perihelion occurs a day later every 58 years due to the phenomenon of precession. So if we wait for about 10,000 years, which is a short time in astronomical terms, perihelion will occur during the northern summer. Then it will be the turn of the people in the northern hemisphere to have to cope with slightly hotter and shorter summers.

To sum up, the seasons and the calendar are linked to the solstices – the longest and shortest days. Having perihelion occur during January in our time, does have a small effect, but has no direct connection to the seasons or the calendar.

Enjoy summer!

7 responses to “The solstice and perihelion and how they relate to summer and the calendar

  • Why is that in the N hemisphere (not checked SH) after the winter solstice, sunset getting later and dawn getting earlier are not evenly incremental ? (Sunset gets later more rapidly than dawn getting earlier)

    • UR MUM, If there was no aphelion or perihelion then Earth’s orbit around the Sun would be perfectly circular. Very few objects in space have circular, or even nearly circular, orbits. Most are elliptical. However, if it were circular then, of course, there would not be the small effect on the seasonal climate mentioned above. And any of the other other long-term climatic affects that depend on the change in ellipticity of Earth’s orbit would also not occur. The precise length of the seasons would change, solar eclipses would be slightly shorter or longer, and there may be numerous other minor effects. Without careful measurement I don’t think any of these effects would be noticable in our day to day lives.

  • This is an interesting and useful introduction, and I do agree that the general populous are unaware of the topic. Of course, the situation in regards influences on Earth’s climate is far more complicated than this.

    One point on the variable distance between the Earth and Sun, is that at the at the present time, the extent of the southern hemisphere having larger area of ocean with the northern hemisphere having much more solid ground. This means that the absorption of heat drawn into the atmospheric system is markedly different between the hemispheres. While on ‘average differences’ are the same in each tropical year, the smaller variations account in some part with larger difference experienced in summer and winter temperature extremes. The result, too, is not constant over the millennia, and especially near the poles, cause differences in so-called ‘solar forcing’

    An eccentric orbit and precession are not the only differences in the order of thousands of years. The so-called obliquity of the ecliptic (the angle of the Earth’s tilt or ‘epsilon’) changes between 22.4 degrees and 24.5 degrees in an averaged 41000 year period. For example, now the tilt is about 23 deg. 26m 16.9s (2000). In 8000BC the ecliptic tilt was 24 deg 20 min. while in 12,000 AD this will reduce to 22 deg 37 min. (The average or middle tilt of 23.5 degree happens to occur today.) In 19,466 AD (plus or minus about 80 years), the tilt will be exactly 23 degrees.

    There is also a consequence to this with precession of the equinoxes, as the cycle is not a circular loop (called the Newcomb Solution), but is a helical spiralling loop. Its 25,000 to 26,000 years in length is also not even a constant rate of change! When the cycle goes 360 degrees, the difference in the spiral can be about is about three-quarter of one degree!

    [General explanations of this inconsistency assumes that the Earth’s axis undergoes precession due the combination of effects of either; Earth’s non-spherical shape, being an oblate spheroid, bulging outward at the equator. Gravitational tidal forces of the Moon and Sun applying torque as they attempt to pull the equatorial bulge into the plane of the ecliptic. Some components of precession is due to the combined action of the Sun and the Moon is called luni-solar precession.]

    When these varying cycles are all merged together, the differences combined to change the climate from cold periods of glaciation to hot warmer and wetter periods.

    On even longer geological timescales, maybe are caused slight differences in solar output, and some have even speculated that the 250 million galactic period of the Sun around the galaxy may have some influence – mostly as intergalactic space changes radiation levels by cosmic rays or the proximity of supernovae. Even rare stellar encounters may influence the Oort Cloud, via perturbations, causing periods of increased comet activity – increasing possibility of collisions with Earth and climate upheavals. (Perhaps in two million years time, Alpha Centauri present passing encounter may cause this very scenario.)

    As a concluding comment, I think the Aboriginal aboriginals have the better means of judging seasons, which depend on observations of the behaviour of the local flora and fauna. Here, depending on where you are in Australia, have up to six seasons. While the astronomical influences are very interesting, they do not relate directly with the human mind, whom are integral part of the interaction and experiences of our climate. This may explain why us ‘urbanites’ are so disconnected from the longer cycles beyond our experience.

    Enough said. Thanks for this story…

  • A very good description. Thanks for that.

    Perhaps you could do one on the moon and its effect on tides. We were talking about this a couple of days ago in the office and someone was wondering how a “spring” tide could occur in summer.

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