Everything you need to know: Autumn equinox 2013

The March equinox signals the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It marks that special moment when the sun crosses the celestial equator going from south to north. In 2013, this equinox will arrive on March 20 at 11:02 Universal Time, or 6:02 a.m. Central Daylight Time for us in the central U.S.

For the remainder of the 21st century (2001-2100), the March equinox will fall on March 20 or 19. Yet it won’t first occur on March 19 until the year 2044, and it won’t happen on March 21 until the year 2102. The March equinox last took place on March 21 in 2007.

A Chinese perspective on the spring equinox
Equinox 2013 comes before sunrise March 20 for most of U.S.
2013 equinox: Sun rises due east and sets due west

If you’re curious (or confused) about the various dates of this equinox, you’re not alone. In a nutshell, it’s because the tropical year, as measured between successive March equinoxes, doesn’t have an even number of days (365.242 days). Our calendar, on the other hand, has an even 365 days in a common year and 366 days in a leap year.

The centennial year 2000 is a leap year, bringing about earlier dates for the March equinox in the 21st century (2001-2100). However, the suppression of the leap year in the centennial year 2100 pushes the dates upward again in the 22nd century (2101-2200).

Here’s what we all know about this equinox. In the Northern Hemisphere, the sun is rising earlier now, and nightfall comes later. Plants are sprouting. Winds are softening. For us in the Northern Hemisphere, people are enjoying the warmer days of spring. South of the equator, autumn begins.

What is an equinox?

The equinox is an event that happens in Earth’s orbit around the sun. Simultaneously, it happens on the imaginary dome of our sky. The imaginary celestial equator is a great circle dividing the sky into Northern and Southern Hemispheres. The celestial equator wraps the sky directly above Earth’s equator. At the equinox, the sun crosses the celestial equator, to enter the sky’s northern hemisphere.

Our ancestors didn’t understand the equinoxes and solstices as events that occur in the course of Earth’s yearly orbit around the sun. But they surely marked today as being midway between the sun’s lowest path across the sky in winter and highest path across the sky in summer.

Today, we know each equinox and solstice is an astronomical event, caused by Earth’s tilt on its axis and ceaseless motion in orbit.

Because Earth doesn’t orbit upright, but is instead tilted on its axis by 23-and-a-half degrees, Earth’s Northern and Southern hemispheres trade places in receiving the sun’s light and warmth most directly. We have an equinox twice a year – spring and fall – when the tilt of the Earth’s axis and Earth’s orbit around the sun combine in such a way that the axis is inclined neither away from nor toward the sun.

At the equinox, Earth’s two hemispheres are receiving the sun’s rays equally. Night and day are approximately equal in length. The word equinox comes from the Latin aequus (equal) and nox (night).
But, since Earth never stops moving around the sun, these days of equal sunlight and night will change quickly.

Where should I look to see signs of the equinox in nature?

The knowledge that spring is here – and summer is coming – is everywhere now, on the northern half of Earth’s globe.

If you live in Earth’s Northern Hemisphere, you’ve likely been noticing the earlier dawns and later sunsets for some weeks now.

Also notice the arc of the sun across the sky each day. You’ll find it’s shifting toward the north. Birds and butterflies are migrating back northward, too, along with the path of the sun.

The longer days bring with them warmer weather. People are leaving their winter coats at home. Trees are budding, and plants are beginning a new cycle of growth. In many places, spring flowers are beginning to bloom.

Does the sun rise due east and set due west at the equinox?

Yes, it does. And that’s true no matter where you live on Earth, because we all see the same sky.
No matter where you are on Earth, you have a due east and due west point on your horizon. That point marks the intersection of your horizon with the celestial equator – the imaginary line above the true equator of the Earth.

At the equinoxes, the sun appears overhead at noon as seen from Earth’s equator, as the illustration at right shows. This illustration (which is by Tau’olunga) shows the sun’s location on the celestial equator, every hour, on the day of the equinox.

That’s why the sun rises due east and sets due west for all of us. The sun is on the celestial equator, and the celestial equator intersects all of our horizons at points due east and due west.

This fact makes the day of an equinox a good day for finding due east and due west from your yard or other favorite site for watching the sky. Just go outside around sunset or sunrise and notice the location of the sun on the horizon with respect to familiar landmarks.

If you do this, you’ll be able to use those landmarks to find those cardinal directions in the weeks and months ahead, long after Earth has moved on in its orbit around the sun, carrying the sunrise and sunset points northward.

So enjoy the 2013 spring equinox on March 20 – an event that happens on our sky’s dome – and a seasonal marker in Earth’s orbit around the sun!

Bottom line: In 2013, the vernal equinox comes on March 20 at 11:02 Universal Time, or 6:02 a.m. Central Daylight Time for us in the central U.S. This post explains the significance of an equinox.

How to translate Universal Time to your time zone

- EarthSky

December Solstice 2012

The day and night sides of Earth at the instant of the December 2012 solstice (11:12 Universal Time on December 21).

Today is the December solstice. Longest night in Northern Hemisphere. Longest Day in Southern Hemisphere.

Late dawn. Early sunset. Short day. Long night. For us in the Northern Hemisphere, the December solstice marks the longest night and shortest day of the year. Meanwhile, the Southern Hemisphere is having its longest day and shortest night.

No matter where you live on Earth’s globe, it’s your signal to celebrate.

When is the solstice where I live?

The solstice happens at the same instant for all of us, everywhere on Earth. In 2012, the December solstice takes place on Friday, December 21 at 5:12 a.m. CST (11:12 UTC). To find the time in your location, you have to translate to your time zone. Check out the following EarthSky article. Just remember: you’re translating from 11:12 Universal Time on December 21.

- Earthsky

Summer Solstice 2012

The summer solstice occurs exactly when the axial tilt of a planet's semi-axis in a given hemisphere is most inclined towards the star that it orbits. Earth's maximum axial tilt to our star, the Sun, during a solstice is 23° 26'. This happens twice each year, at which times the Sun reaches its highest position in the sky as seen from the North or South Pole respectively.

The summer solstice is the solstice that occurs in a hemisphere's summer. In the Northern Hemisphere this is the Northern solstice, in the Southern Hemisphere this is the Southern solstice. Depending on the shift of the calendar, the summer solstice occurs some time between December 20 and December 23 each year in the Southern Hemisphere and between June 20 and June 22 in the Northern Hemisphere in reference to UTC.

Though the summer solstice is an instant in time, the term is also colloquially used like Midsummer to refer to the day on which it occurs. Except in the polar regions (where daylight is continuous for many months), the day on which the summer solstice occurs is the day of the year with the longest period of daylight.

Worldwide, interpretation of the event has varied among cultures, but most have held a recognition of sign of the fertility, involving holidays, festivals, gatherings, rituals or other celebrations around that time.

The solstices, together with the equinoxes, are connected with the seasons. In many cultures the solstices mark either the beginning or the midpoint of winter and summer.

The term solstice can also be used in a broader sense, as the date (day) when this occurs. The day of the solstice is either the longest day of the year (in summer) or the shortest day of the year (in winter) for any place outside of the tropics.

The 2012 December solstice takes place on Friday, December 21 at 11:12 Universal Time. That’s 5:12 a.m. Central Standard Time. Happy solstice!

-  Wikipedia

Does the sun rise due east and set due west at the equinox?

Image credit: James Jordan

Yes, it does. And that’s true no matter where you live on Earth, because we all see the same sky.

No matter where you are on Earth, you have a due east and due west point on your horizon. That point marks the intersection of your horizon with the celestial equator – the imaginary line above the true equator of the Earth.

At the equinoxes, the sun appears overhead at noon as seen from Earth’s equator. That’s the definition of an equinox: it’s when the sun crosses the celestial equator, as seen in Earth’s sky.

That’s why the sun rises due east and sets due west for all of us on the day of an equinox. The sun is on the celestial equator, and the celestial equator intersects all of our horizons at points due east and due west.

This fact makes the day of an equinox a good day for finding due east and due west from your yard or other favorite site for watching the sky. Just go outside around sunset or sunrise and notice the location of the sun on the horizon with respect to familiar landmarks.

If you do this, you’ll be able to use those landmarks to find those cardinal directions in the weeks and months ahead, long after Earth has moved on in its orbit around the sun, carrying the sunrise and sunset points southward.

- EarthSky

Everything you need to know: September equinox 2012

Image credit: Przemyslaw Idzkiewicz.

The 2012 September equinox comes on September 22, at 9:49 a.m. CDT (14:49 Universal Time).

What is an equinox?

The earliest humans spent more time outside than we do. They used the sky as both clock and calendar. They could easily see that the sun’s path across the sky, the length of daylight, and the location of the sunrise and sunset all shift in a regular way throughout the year.

Our ancestors built the first observatories to track the sun’s progress. One example is at Machu Picchu in Peru, where the Intihuatana stone, shown at right, has been shown to be a precise indicator of the date of the two equinoxes and other significant celestial periods. The word Intihuatana, by the way, literally means for tying the sun.

Today, we know each equinox and solstice is an astronomical event, caused by Earth’s tilt on its axis and ceaseless orbit around the sun.

Because Earth doesn’t orbit upright, but is instead tilted on its axis by 23-and-a-half degrees, Earth’s Northern and Southern Hemispheres trade places in receiving the sun’s light and warmth most directly. We have an equinox twice a year – spring and fall – when the tilt of the Earth’s axis and Earth’s orbit around the sun combine in such a way that the axis is inclined neither away from nor toward the sun.

Earth’s two hemispheres are receiving the sun’s rays equally now. Night and day are approximately equal in length. The name ‘equinox’ comes from the Latin aequus (equal) and nox (night).

So enjoy the 2012 equinox – September 22 or 23 depending on your time zone – a seasonal signpost in Earth’s orbit around the sun!

- EarthSky

A "TOUCH OF SPRING" in the streets of Durban

Images: Tabebuia in the streets of Durban - Obs. Dee de Souza

Winter Solstice: Start of winter in South Africa

The first day of the Season of Winter. On this day (JUNE 21 in the southern hemisphere) the Sun is nearest north and the length of time between Sunrise and Sunset is the shortest of the year.

If you’re in the Southern Hemisphere, the June solstice is your signal to celebrate winter. For us, this solstice marks the shortest day of the year. Late dawns. Short days. Early sunsets. Long nights.

Enjoy celebrating the start of winter!!

Everything you need to know: June solstice 2012

If you’re in the Northern Hemisphere, the June solstice is your signal to celebrate summer. For us, this solstice marks the longest day of the year. Early dawns. Long days. Late sunsets. Short nights. The sun at its height each day, as it crosses the sky. Meanwhile, south of the equator, winter begins.

When is the solstice where I live? It is 23:09 UTC (6:09 p.m. CDT in the U.S.) when this solstice takes place on Wednesday, June 20, 2012. See on the globe of Earth how it’s afternoon in the U.S. and morning for east Asia and Australia at this solstice? It’s high noon over the Pacific Ocean, just a bit west of the Hawaiian Islands.

The solstice happens at the same instant for all of us, everywhere on Earth. To find the time of the solstice in your location, you have to translate to your time zone.

What is a solstice? Ancient cultures knew that the sun’s path across the sky, the length of daylight, and the location of the sunrise and sunset all shifted in a regular way throughout the year.

They built monuments, such as Stonehenge, to follow the sun’s yearly progress.

Today, we know that the solstice is an astronomical event, caused by Earth’s tilt on its axis and its motion in orbit around the sun.

Because Earth doesn’t orbit upright. Instead, our world is tilted on its axis by 23-and-a-half degrees, Earth’s Northern and Southern Hemispheres trade places in receiving the sun’s light and warmth most directly.

At the June solstice, Earth is positioned in its orbit so that our world’s North Pole is leaning most toward the sun. As seen from Earth, the sun is directly overhead at noon 23 1/2 degrees north of the equator, at an imaginary line encircling the globe known as the Tropic of Cancer – named after the constellation Cancer the Crab. This is as far north as the sun ever gets.

All locations north of the equator have days longer than 12 hours at the June solstice. Meanwhile, all locations south of the equator have days shorter than 12 hours.

Where should I look to see signs of the solstice in nature? Everywhere.

For all of Earth’s creatures, nothing is so fundamental as the length of the day. After all, the sun is the ultimate source of almost all light and warmth on Earth’s surface.

If you live in the Northern Hemisphere, you might notice the early dawns and late sunsets, and the high arc of the sun across the sky each day. You might see how high the sun appears in the sky at local noon. And be sure to look at your noontime shadow. Around the time of the solstice, it’s your shortest noontime shadow of the year.

If you’re a person who’s tuned in to the out-of-doors, you know the peaceful, comforting feeling that accompanies these signs and signals of the year’s longest day.

Why celebrate the solstice? Cultures universally have had markers, holidays, and alignments – all related to the solstice.

It has been universal among humans to treasure this time of warmth and light.

For us in the modern world, the solstice is a time to recall the reverence and understanding that early people had for the sky. Some 5,000 years ago, people placed huge stones in a circle on a broad plain in what’s now England and aligned them with the June solstice sunrise.

We may never comprehend the full significance of Stonehenge. But we do know that knowledge of this sort wasn’t isolated to just one part of the world. Around the same time Stonehenge was being constructed in England, two great pyramids and then the Sphinx were built on Egyptian sands. If you stood at the Sphinx on the summer solstice and gazed toward the two pyramids, you’d see the sun set exactly between them.

How does it end up hotter later in the summer, if June has the longest day? People sometimes ask: “If the June solstice brings the longest day, why do we experience the hottest weather in late July and August?”

This effect is called “the lag of the seasons.” It’s the same reason it’s hotter in mid-afternoon than at noontime. Earth just takes a while to warm up after a long winter. Even in June, ice and snow still blanket the ground in some places. The sun has to melt the ice – and warm the oceans – and then we feel the most sweltering summer heat.

Ice and snow have been melting since spring began. Meltwater and rainwater have been percolating down through snow on tops of glaciers.

But the runoff from glaciers isn’t as great now as it’ll be in another month, even though sunlight is striking the northern hemisphere most directly around now.

So wait another month for the hottest weather. It’ll come when the days are already beginning to shorten again, as Earth continues to move in orbit around the sun, bringing us closer to another winter.

And so the cycle continues.

Bottom line: The 2012 June or summer solstice takes place on Wednesday, June 20, 2012 at 23:09 Universal Time (6:09 p.m. CDT). This solstice – which marks the beginning of summer in the Northern Hemisphere – marks the sun’s most northerly point in Earth’s sky. It’s an event celebrated by people throughout the ages.

- Earthsky