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.
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.