Many people believe that Earth is closer to the Sun in the summer and that is why it is hotter. And, likewise, they think Earth is farthest from the Sun in the winter. Show Although this idea makes sense, it is incorrect. It is true that Earth's orbit is not a perfect circle. It is a bit lop-sided. During part of the year, Earth is closer to the Sun than at other times. However, in the Northern Hemisphere, we are having winter when Earth is closest to the Sun and summer when it is farthest away! Compared with how far away the Sun is, this change in Earth's distance throughout the year does not make much difference to our weather. There is a different reason for Earth's seasons. Earth's axis is an imaginary pole going right through the center of Earth from "top" to "bottom." Earth spins around this pole, making one complete turn each day. That is why we have day and night, and why every part of Earth's surface gets some of each. Earth has seasons because its axis doesn't stand up straight. But what caused Earth to tilt? Long, long ago, when Earth was young, it is thought that something big hit Earth and knocked it off-kilter. So instead of rotating with its axis straight up and down, it leans over a bit. By the way, that big thing that hit Earth is called Theia. It also blasted a big hole in the surface. That big hit sent a huge amount of dust and rubble into orbit. Most scientists think that that rubble, in time, became our Moon. As Earth orbits the Sun, its tilted axis always points in the same direction. So, throughout the year, different parts of Earth get the Sun’s direct rays. Sometimes it is the North Pole tilting toward the Sun (around June) and sometimes it is the South Pole tilting toward the Sun (around December). It is summer in June in the Northern Hemisphere because the Sun's rays hit that part of Earth more directly than at any other time of the year. It is winter in December in the Northern Hemisphere, because that is when it is the South Pole's turn to be tilted toward the Sun. Earth's lopsided orbitEarth's perihelion (point closest to Sun) = 91,400,000 miles from Sun Earth's aphelion (point farthest from Sun) = 94,500,000 miles from Sun While that is a difference of over 3 million miles, relative to the entire distance, it isn’t much. And, believe it or not, aphelion (when Earth is farthest from the Sun) occurs in July, and perihelion (when we are closest) occurs in January. For those of us who live in the Northern Hemisphere where it's summer in July and winter in January, that seems backwards, doesn't it? That just goes to prove that Earth's distance from the Sun is not the cause of the seasons. Related Resources for Educators Seasons (Educator Guide to go with Seasons Spotlite video)
Good gardeners learn to time their work according to the seasons, but when do the seasons begin and end? There are different ways to divide up a year into seasons, changing when each actual season starts. Seasons can be meteorological or more traditionally astronomical, so which is which and how do you convert between the two? Confusion about seasons doesn’t end there! Ever read a gardening book written on the other side of the world that talks about what month to do something in the garden, rather than what season, leaving you confused? Ideally it would be nice if gardening books were written to be more universal, but often they’re not so a way of converting months to seasons and translating northern hemisphere seasonal references in the southern hemisphere and vice versa in invaluable. I’ve always wanted a quick reference guide for this purpose so I created a simple conversion table for gardeners which will make sense of overseas gardening books and local seasonal timing. Feel free to share! Gardening Season Timing and Conversion ChartWant to convert seasons from say a northern hemisphere gardening book, US or UK for example, and live in the southern hemisphere, say Australia? It’s easy, just look up the Meteorological season for the month in the table below to find out which season and part of that season that month refers to. Note: click on graphic above to enlarge and save image, or download the PDF version of the gardening season timing and conversion chart for printing The Difference Between Meteorological or Astronomical SeasonsBefore anyone gets the wrong impression, the title says astronomical and not astrological!!! Some people mix these two words up, which brings an amusing little anecdote to mind. When I was working in the corporate world in a technical area many eons ago, I overheard a male colleague blurt out “I don’t believe in all that astronomy stuff” to which a female colleague sitting nearby wittily responded “So you don;t believe there’s a sun in the centre of our galaxy with planets revolving around it?” Quite embarrassed, the male colleague sheepishly replied “No, I mean the other one, you know what I mean…” What are Astronomical Seasons?Traditionally, seasons began at the solstices and equinoxes. Solstices are the longest and shortest days of the year, so the midsummer solstice is the longest day of the year and the and midwinter solstice is the shortest day of the year. The equinoxes are the days when the length of day and night are exactly equal, such as what happens in the spring and autumn equinoxes. These solstices and equinoxes occur around the third week of the month, and the days change from year to year, dependent upon the positions of the Earth in relation to the sun, as explained below. So, in summary, Astronomical Seasons change (begin) at the equinoxes and solstices, the dates when they start are based on the position of Earth in relation to the sun. If you want to understand how astronomical seasons work, it’s not that complicated. The key is the tilt of the Earth’s axis in relation to the sun. If we look at a Northern Hemisphere example, when the Earth’s axis is tilted furthest towards the sun, the sun’s light shines more directly on the northern latitudes (northern hemisphere), producing the astronomical summer, which occurs approximately on June 20-22. (This is also the time of the winter solstice in the southern hemisphere). (Image source: public domain image by Przemyslaw Idzkiewicz)
(Image source: public domain image by Przemyslaw Idzkiewicz) The equinoxes are fairly easy to understand, they occur when the Earth is tilting halfway between the summer and winter extremes and the sun’s light shines more directly on the equator, and the length of night and day are nearly equal, producing the astronomical spring approximately on March 20 and astronomical autumn approximately on September 23 in the northern hemisphere. What are Meteorological Seasons?There’s an even simpler way we can divide up the seasons – we can divide up the year into four 3-month periods which have similar temperatures, and we call these meteorological seasons. The meteorological seasons begin at the beginning of a particular month, and end three months later at the end of the month. Meteorologists (weather scientists) implemented this system which deals with temperatures over whole months (rather than astronomical part months) to allow them to more easily compare weather patterns from one season to another. So, in summary, Meteorological Seasons change (begin) every 3 months, the dates when they start are based on groups of whole months that are similar in temperatures. Using meteorological seasons for weather seasonal comparisons is easier because temperatures are more consistent across a season this way. Using astronomical seasons is more difficult because there is a seasonal lag, a delay between the time the astronomical season changes and the seasonal temperatures settling in. We can see from our season conversion chart that meteorological seasons and astronomical seasons don’t neatly coincide or marry up, they’re out by approximately one month. In other words, the spring equinox may occur on March 20, which is closer to the start of April, but real spring temperatures will arrive earlier at the start of March in the northern hemisphere. In the southern hemisphere, the spring equinox will occur on September 23, which is closer to the start of October, but real spring temperatures will arrive earlier at the start of September. Which Set of Seasons Do We Use?Modern gardening calendars typically just use the meteorological seasons, so each season begins at the start of a certain month.
The astronomical seasons are the more traditional way of defining seasons, so if you’re planting by the moon (lunar calendar planting) or practising biodynamic gardening where the equinoxes and solstices are important, then this way of defining seasons may be more useful. |
What season would it be in the Northern Hemisphere?
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