The seasons are governed by the tilt of the Earth’s axis in space as it journeys around the Sun in a year. When the South Pole of the Earth is tilted towards the Sun, this is our Summer. Six months later, when the South Pole is tilted away from the Sun, it's our Winter. In between these we have Autumn and Spring.
The Sun and the seasonsArtist: Frey Micklethwait. Source: Museum Victoria.
Temperatures on our planet are not determined by the distance of the Earth from the Sun. Rather it is the angle of the Sun’s rays striking the Earth. In Summer, the Sun is high in the Sky and the rays hit the Earth at a steep angle. In winter, the Sun is low in the Sky and the rays strike the Earth at a shallow angle.
The seasons don’t begin on one day and finish on another. That's because our orbit around the Sun is continuous. It actually takes quite some time for the Earth to heat up or cool down, and that’s why the seasons change gradually.
In some parts of the world, such as Australia, seasons begin on the first day of a particular calendar month - in March for Autumn, June for Winter, September for Spring and December for Summer. In other countries such as Britain, it’s accepted that the seasons begin on the dates that the Earth passes four special points in its orbit about the Sun.
2014 September 23, 12:292015 September 23, 18:20 2016 September 23, 00:21
The Sun in springArtist: Frey Micklethwait. Source: Museum Victoria.
On the day of the Spring Equinox, the Earth’s poles are the same distance from the Sun. In Melbourne, the Sun rises due east, sets due west and gets to 52° above the horizon at noon. On this day there are roughly 12 hrs of day and 12 hrs of night.
2014 December 22, 10:032015 December 22, 15:482016 December 21, 21:44
The Sun in summerArtist: Frey Micklethwait. Source: Museum Victoria.
On the day of Summer Solstice, the Earth’s south pole is tilted towards the Sun. The Sun rises south of east, sets south of west and reaches 75 1/2° above the horizon at noon. This is, usually, the longest day of the year.
2014 March 21, 03:572015 March 21, 09:452016 March 20, 15:30
The Sun in autumnArtist: Frey Micklethwait. Source: Museum Victoria
On the day of the Autumn Equinox, the Earth’s poles are the same distance from the Sun. The Sun rises due east, sets due west and reaches 52° above the horizon at noon. There are roughly 12 hrs of day and 12 hrs of night.
2014 June 21, 20:512015 June 22, 02:382016 June 21, 08:34
The Sun in winterArtist: Frey Micklethwait. Source: Museum Victoria.
On the day of Winter Solstice, Earth’s south pole is tilted away from the Sun. The Sun rises north of east, sets north of west and reaches 28 1/2° above the horizon at noon. This is, usually, the shortest day of the year.
Leigh: the shortest day in the year is the Winter Solstice whose dates are mentioned in this info-sheet. Further details about the sun's movements can be found over at this sheet, too.
The Discovery Centre can answer questions that relate to Museum Victoria's Collection and Research areas, including Science. If you have specific questions relating to astronomy and meteorology, such as the solstice, you can ask one of our experts on staff by contacting the Discovery Centre directly.
An edited version of our Planetarium staff's response to Stephen's question is as follows:
The Summer Solstice is today the 22nd Dec at 04:47 Australian Easter Summer Time, or if you look at international sites you will see it listed as being on the 21st Dec. at Universal Time 17:47.
The actual time difference between Wollongong and Melbourne is immaterial. It is based on the precise time that the angle of the Earth’s tilt is most inclined toward or away from the Sun.
Compasses indicate magnetic north or south as they react to the Earth’s magnetic field but that’s offset with respect to our planet’s rotational axis (what most call geographic or true north/south). The direction of magnetic north or south is therefore slightly different. It's possible to find south using the position of the Sun in the sky but it's hard to be really accurate. A reasonable result can be got by observing the motion of the Sun across the sky. Wait for local noon by using a clock on standard time (simpler than having to allow an hour for daylight savings). At that time the Sun will be at its highest point above the horizon, so face the Sun and south will be behind you if you are in the Southern Hemisphere (or north will be behind you if in the Northern Hemisphere). Better still, use a sundial or a tall pole that will throw a shadow towards south at local noon (or towards north if in the Northern Hemisphere). You can then compare compass readings at the building and some well away from it with your noon observing of the Sun. Overall you should get a pretty good facing direction. These websites will give more information:
Hi John - The equinoxes occur when the Sun is right across the celestial equator, and so day and night are of the same length for all observers on Earth. The equinoxes happen in March and September. In June and December are the solstices, when the Sun reaches its northernmost (in June), or southernmost (in December) point in the sky, and appears to stop (in its north-south movement) before reversing and heading back in the other direction.
The date and time of the equinoxes and solstices change slightly because the Earth's orbit around the Sun is not precisely 1 year. The equinoxes and solstices happen when the Earth reaches a precise point in its orbit around the Sun.
However the beginning of the year does not happen at the exact same point every year. We count years as being either 365 or 366 days long, but the orbit actually takes about 365.242 days to go from one March equinox to the next.
Thus in a non-leap year, the equinox will be about 0.242 days (approximately 5 hours 50 minutes) later than the previous equinox, while in a leap-year it will be 0.758 days (18 hours 10 minutes) earlier. Note that these figures aren't exact, and the combined effects of gravity from the Moon, the Sun and the other planets also cause a few minutes variation from year to year.
That's correct - if the Earth's axis was not tilted, we would not experience the seasons as we understand them, since the Sun's rays would not be striking the surface of the Earth at such steep angles. If the Earth was not tilted, the Sun's rays would strike the Earth at a uniform angle.
Hi Felicity - It is not true that there are two summer solstices at these locations. The summer solstice happens at a specific point in the Earth’s orbit. What is true is that the Sun will appear directly overhead twice a year in these locations. However this is not on the day of the summer solstice. On this day the day length is longest, and the Sun appears the most southerly, and this will only happen once each year no matter where you are.
We do not believe there is any relation between the equinox, and the weather or tides. However it possibly is more windy between the seasons because that is the pattern in that part of the world at that time, thus, a coincidence.As far as the effect on the spring tides, it should be pointed out that the “Spring” does not refer to the season in this instance, it refers to an earlier meaning, to spring forward or to rise. This is when the Sun and Moon align to give a higher than normal tide. The opposite of spring tide is neap tide when it is lower than usual.
Hi Ronnie, there is not actually an 'official start date' for seasons. When a season starts on a certain day it is only by convention. There is no government act or universally accepted scientific definition for what constitutes a season.Most countries popularly use the equinox and solstice dates to 'start' the seasons' but meteorologists around the world start counting the seasons in the same way that Australians do - on March 1st, June 1st etc
Hi JB, the Summer and Winter Solstices are a moment in time when the Sun is at its most northern or highest in the sky and most southern or lowest in the sky extremes. This year this will occur on the 21 June at 11:28 UTC time and at 21st December at 23:38 UTC time. So in answer to your question about how long a solstice is, it is just a point in time. Have a look at our infosheet on the path of the sun for more information.
Hi Diana, the bright side of the Moon always faces the Sun of course as that is why it is bright. From Melbourne this would generally mean that the bright side is facing north. The axis of the earth has not appreciably changed because of the recent earthquakes, but may have shortened our day by about a millionth of a second. It certainly would not have changed the sun angle onto the moon.
Hi Mary, all our times and charts are for Melbourne at 37° 49’ 00 S and 144° 58’ 00 E. The times are local times adjusted for daylight savings. Now, as far as solstice and equinoxes go, they are all at the same Universal Time or GMT, and are given in local times.
The following web address should assist with any other queries you may have: http://www.ga.gov.au/geodesy/astro/moonrise.jsp another good site is : http://aa.usno.navy.mil/
Hi Alan, when seasons officially start and stop is a local convention that countries and communities adopt, and it depends on your latitude as well. If you are in the tropics then there are only two seasons: the wet and the dry. Our aboriginals have 3 to four seasons again depending on your latitude. See this great web site: http://www.bom.gov.au/iwk/climate_culture/Indig_seasons.shtml
The Summer Solstice in Melbourne is generally not the hottest time of the year as the earth hasn’t heated up fully, and is usually mid to end January. So you could say this is mid- summer in Melbourne. Season times as can be seen are purely local convention and are based about the solstice.
Hi Pauline, this link should give you the answers you are looking for.
Hi Don, the Planetarium staff have said that the dates on this link are correct. The Spring Equinox occurs at 00:49am on the 23rd Sept. 2012 but on the other side of the Earth it occurs on the 22nd September.
Hi Nat - All places on Earth experience the summer and winter solstice. The tilt of the Earth means that on the solstice days the rays of the sun strike either the latitudes defined by the Tropic of Cancer or The Tropic of Capricorn. On the northern hemisphere winter solstice there is no light above 66.5 degrees north (Polar Circle) but it is the summer solstice for the southern hemisphere where there is 24 hours of sunlight below 66.5 degrees south. If the earth did not tilt then there would be no solstice and no seasons (Swinburne University).
The Melbourne Museum has information sheets that help explain the path of the sun and the tilt of the Earth:
The path of the sun
The sun and the seasons
Hi Luke, we chatted with the Planetarium staff for you, and they have found a great website that will settle the debate between you and your boss. According to http://aa.usno.navy.mil/data/docs/AltAz.php, for a short period around the longest day, 21st December, in Cairns, you would be in the shade, but for most of the year you would be in the Sun. Sorry Luke, I think your boss is right!
We're not sure what it is you're asking - could you please clarify your question?
For any latitude the Sun’s rising and setting points on the horizon, and its daytime path across the sky, haven’t changed in their seasonal pattern in any meaningful way since humans began observing such things. Of course, the rising and setting positions and the Sun’s path all do shift gradually during the year but that regular seasonal/annual cycle is due to the steady tilt of Earth’s axis at 23.5 degrees. There is, however, a very small change to the tilt over tens of thousands of years. This is due to the gravitational influences of the Moon and other planets, and minor changes to the elliptical shape of Earth’s orbit around the Sun. That small (“wobble”) effect wouldn’t be noticeable in a human lifetime. A major earthquake can make a very tiny change to the tilt but that would not make a noticeable difference either to where the sun rises, sets, or its path in the sky.
Hi Ernemef, apologies for the delay in replying to your query. With Giza at latitude 29.9 degrees North, the Sphinx does face directly east at summer solstice. That is, at right angles to earth’s north-south geographical axis, or north geographical pole. For more info see:
Kikipanda; we're not sure if your question relates to the reversal of the equinixes and solstices between the northern and southern hemispheres, or if you are asking about the apparent timing differences of the event at different regions of the globe. Or, if you are curious about the slight timing differences from year to year.
In any case, each equinox and solstice event occurs only once and at one time (two of each per year), and depending on which side of the equator you are located determines what sort of equinox or solstice it is called, as seasons are reversed between hemispheres. For example, the Summer Solstice in Australia - the longest day - is the same event as the Winter Solstice in the UK - their shortest day. Astronomically its all the same event, but the position relative to the equator determines the season in which the event occurs, which therefore determines the name given to it.
The exact timing of solstices and equinoxes is a single occurence/event in each case - the timing is usually shown in UTC, which is functionally the same as Greenwich Mean Time. Often the timing are calculated to the relevant time zone for clarity as shown in the page above, where it is shown in in Australian Eastern Standard Time (AEST) of Australian Eastern Daylight Savings Time (AEDT) - these are the timings of these events as they occur in Melbourne.
Interesting idea Sandy, but according to the Bureau of Meteorology, Australian seasons are interpreted via climatic rather than astronomical events. Working on the principle that the three coldest months being regarded as winter, the three hottest as summer, and the three month transitional seasons either side of these extremes being Spring and Autumn respectively, this is how the first day of every third month coincides with the first day of the season.
Evidently the climate is, in turn, heavily influenced by solstices and equinoxes due to the angle of the sun, but the exact dates of these astronomical events aren't used to correlate the official onset of seasons here in Australia.
I'm not sure of any possible commercial advantage of distancing the official onset of Summer from Christmas, especially as shops are already gearing up for Christmas retailing now, and it's only September; the commerciality of Christmas seems indifferent to the seasons.
One thing to note is that there is not really an "official start date" for seasons at all. When we talk about a season starting on a certain day it is only by convention. There is no government act or universally accepted scientific definition for what constitutes a season.
While most countries popularly use the equinox and solstice dates to "start the seasons" there is no particular reason why this convention needs to be followed. Meteorologists around the world, for example, start counting the seasons in the same way that Australians do - on March 1, June 1, September 1, and December 1.
Yes, basically that’s correct. For matching northern/southern latitudes the hours of daylight, the length of the day, follows the same seasonal pattern, although opposite in one hemisphere compared to the other (winter in one while summer in the other).
However, a check on any atlas or globe will show that no part of Australia has an equivalent southern latitude to UK/Northern Europe. The closest would be Macquarie Island halfway between New Zealand and Antarctica, which is similar to Cape Horn, the most southerly part of South America. In fact, most of the southern latitude that matches UK/North Europe is open ocean.
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