Daily Bulletin

The Conversation

  • Written by Tanya Hill, Honorary Fellow of the University of Melbourne and Senior Curator (Astronomy), Museum Victoria

We’ve just passed another winter solstice. Wednesday June 21 was the shortest day of the year. I live in Melbourne, so we had just 9 hours and 32 minutes of daylight, and it was dark and grey, so we certainly felt the lack of sunlight.

For those up north and closer to the equator the shortest day is not so extreme. For example, Brisbane had 10 hours and 24 minutes of daylight on Wednesday, almost an hour more than Melbourne. No wonder southerners head north for winter.

Traditionally, the solstice marks the time when the Sun “stands still”.

From our vantage point on Earth, the Sun is changing directions. At 2:24pm June 21, it reached its furthest north for the year and then started heading south.

If you’re like me, you might find that statement a little confusing. What it means is that the Sun is now moving higher in our northern sky, which course means it’s moving southward.

image Still no morning Sun We may have reached our shortest day, but unfortunately it will be a few more weeks before our mornings get any brighter. In fact, sunrise will shift slightly later (by a couple of minutes) and it won’t be until July that the trend will start to shift. Bad news indeed for those of us who struggle to get going in the morning. image But our days are still getting longer, just the extra daylight is added to our afternoons, not our mornings. It’s a pattern that happens around the time of the solstice. At the winter solstice, the earliest sunset (or shortest afternoon), happens first, then the solstice (shortest day), followed by the latest sunrise (or shortest morning). It works in the opposite way for the summer solstice in December. The earliest sunrise comes first (or longest morning), then the solstice (longest day), then the latest sunset (longest afternoon). What’s in a day? While our clocks mark out an equal 24 hours to every day, the Sun is not so steady. When you take a photo of the Sun at the same time every day, not only do you see it move higher and lower in the sky, but it also appears “later” or “earlier” in the east-west direction. image Our inconstant Sun - an analemma is made when you take a photo of the Sun at the same time every day. A solar day is the time it takes for the Sun to return to due north (or local noon) each day and it is constantly changing in length. Not because of the the Earth’s rotation, which is really very constant (to the order of a millisecond). Every 23 hours and 56 minutes, the Earth rotates once on its axis. But as the Earth rotates it also moves along its orbit around the Sun. After 23 hours and 56 minutes, the Earth has moved far enough along that it needs a further 4 minutes, on average, to realign itself to the Sun. image The extra minutes of the solar day: Position 1 - the Earth is pointing towards the Sun; Position 2 - the Earth’s completes one rotation; Position 3 - the Earth must rotate a little further to face the Sun again. Chris828/wikipedia The key word here is average. In February, May, June and July a solar day can equal 24 hours. But around the autumnal equinox in March and the spring equinox in September, the solar day is about 20 seconds less than 24 hours, and at the solstices, the solar day is slightly more than 24 hours. Turning, turning, turning To understand what’s going on, we need to reframe the Earth’s movement. Let’s suspend reality for a moment and imagine how things would work if we switch from a Sun centred view to an Earth centred one. Since the Earth is tilted by 23.5 degrees, let’s position the Earth upright and place the Sun’s orbit on a 23.5 degree tilt. image It often helps if you suspend reality and consider an Earth-centric view. Museums Victoria The solstices are now obvious. They are the moments when the Sun reaches its most northern or southern points. You can also see the why the Earth’s tilt (seen in the diagram as the tilt of the Sun’s orbit) causes the seasons. When the Sun hits its northern most point, it shines down on the northern hemisphere bringing the long days of summer. While here in the south, as the Earth rotates on its axis, it’s the nights that are long and our winter days are short. When the Sun crosses the Earth’s equator it is the time of the equinox (or equal day-night). The “Sun’s orbit” near the equator is relatively steep. The Sun is mostly moving north-south with only a small fraction of its movement in the east-west direction (or parallel to the equator). And because the Sun doesn’t “move” very far in the east-west direction, the Earth doesn’t need to rotate as much for the Sun to return to due north and complete a solar day. That’s why the solar day is less than 24 hours at the time of the equinox. But at the extremes of the “Sun’s orbit” the rate of movement in the north-south direction slows and most of the Sun’s movement is now east-west or parallel to the Earth’s equator. At these times, the Earth has to rotate even further to bring the Sun back to due north, and hence the solar day is longer than 24 hours around the time of the solstices. The tilt and the solar day So there are two things going on here. As we move towards winter, the tilt of the Earth makes the days grow shorter. This naturally brings later sunrises and earlier sunsets. But as we approach the solstice, the second effect kicks in - the solar day starts getting longer. The Earth has to rotate more to bring the Sun back into place and this shifts both the sunrise and sunset progressively later. It pushes the time of latest sunrise to occur after the solstice and that’s why we have this wait to see more of the morning Sun. It also means that the time of earliest sunset must happen before the solstice. image Sunrise over the Melbourne CBD. Michael Sale/flickr At the summer solstice, it all plays out in the opposite way. As we move through spring, the tilt of the Earth makes the days grow longer - we have earlier sunrises and later sunsets. But once again as the summer solstice nears, the lengthening solar day kicks into action, pushing both sunrise and sunset to happen later. It pushes the latest sunset to occur after the summer solstice, while earliest sunrise must occur before the solstice. Of course, when we are basking in the summer Sun we don’t pay quite as much attention. So just hang in there a little longer. We’ve made it past the shortest day and eventually the lengthening daylight will bring us brighter mornings.

Authors: Tanya Hill, Honorary Fellow of the University of Melbourne and Senior Curator (Astronomy), Museum Victoria

Read more http://theconversation.com/why-the-sunrise-is-still-later-after-the-winter-solstice-shortest-day-77628

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