The vernal equinox, which marks the first day of the northern Spring season, occurs at 7:02 a.m. EDT on March 20, 2013.
Image: Starry Night Software
The vernal equinox, which occurs at 7:02 a.m. EDT today (March 20), is probably the most important astronomical event of the year.
Although popularly described as "the first day of spring" in the Northern Hemisphere, the annual event is actually much more important than that. More than any other event, it marks the beginning of the astronomical year.
For something so important in astronomy, it’s surprising that it is something that you cannot observe in any way in the night sky. That’s because it involves the sun, which floods the sky with light and makes it impossible to see what is actually happening.
Modern planetarium software enables us to dim the sun, so that we can see what it is doing in the context of the planets around it and the stars behind it. [6 Signs that Spring Has Sprung]
In the graphic accompanying this story, the celestial equator (located directly above the Earth’s equator) is marked by a red line. Crossing it at an angle is a green line which marks the ecliptic, the path which the sun follows in front of the background stars. This is tilted at an angle because the Earth’s axis of rotation is at an angle, 23.5 degrees.
Notice the months marked along the ecliptic: these indicate the sun's position. February is below and to the right; April is above and to the left. The sun sits on March 20 right in the center. So the sun appears to be moving from right to left across the chart.
In reality, of course, it is the Earth that is moving, and not the sun, but from our viewpoint on Earth it seems just the opposite.
What happens on March 20 is that the sun moves from south of the celestial equator to north of the equator. The days, which have been getting longer since winter solstice on Dec. 21, now equal the nights in length, which is why this is called the "equinox" — Latin for "equal nights."
From now until the summer solstice on June 21, the days will gradually get longer because the sun will be higher in the sky, giving more hours of daylight.
More hours of sun means more warming by the sun, so the days will also get warmer. Also the sun's rays will be striking the northern hemisphere from a more acute angle, so that the same amount of solar radiation is concentrated in a smaller area, amplifying the heating factor.
The graphic shows the constellations in their modern positions. The point at which the equator and the ecliptic cross, traditionally called "the first point in Aries," actually lies today in the constellation Pisces. Aries, where it used to lie, is now up in the upper left corner of the chart.
Similarly, the summer equinox now puts the sun in Sagittarius, off the chart to the lower right, even though it gives its name to the "tropic of Capricorn." One of the differences between astronomy and astrology is that astronomy uses the current positions of the sun and planets in the sky, while astrology uses historical positions which have been out-of-date for thousands of years.
Because the sky has been darkened by the software, we can see how several planets are gathered on the far side (behind) the sun: Uranus, Mars, and Venus. Above the sun and to its left (east) is Comet Pan-STARRS. Below and to the left is the distant dwarf planet Eris, hovering in the far reaches of the solar system.
How did ancient peoples observe and measure the date of the vernal equinox, if it is blocked by the sunlit sky?
They discovered clever ways of marking the rising and setting points of the sun. In some cases they built monuments to refine these measurements. Stonehenge, in southern England, is one of the most spectacular examples of these equinox calendars.
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9 Comments
Add CommentIn Australia ( Latin for'Southern lands'), after the hottest ever summer, we are breathing a sigh of relief at our autumnal equinox time; it means that the long spell of 35 -plus degrees Centigrade- will be broken at last,
Reply | Report Abuse | Link to thiswith the Easter feast beckoning soon- with long-awaited autumn rains-the first in months.
We are truly 'Antipodes' ( Greek for'Down-Under Footers')
with North, not South, being our sunny side up, and the moon waxing and waning the other way round!
It would be nice if there were a link to a bigger version of the software image in question, instead of leaving us to squint at the blurry thumbnail.
Reply | Report Abuse | Link to this"Although popularly described as "the first day of spring"."-it is best called The First Point ofAries.
Reply | Report Abuse | Link to thisI remember that from about 60 years ago.
Now where are my specs?
If you believe the hottest ever claim for Australia you need to expand your reading. Try this: http://joannenova.com.au/
Reply | Report Abuse | Link to thisHottest summer record in Australia? Not so, says UAH satellite data...
The satellite data shows that the summer of 2012-2013 was close to ordinary, compared with the entire satellite record going back to 1979. Not a record. Not even extreme?
The UAH data that you are referring to was published by Roy Spencer, a creationist who doesn't believe in human evolution or human induced climate change. I am not saying that his data is false, but I would take it with a grain of salt considering his anti-scientific religious views.
Reply | Report Abuse | Link to thisLook at the data. Not the messenger. That is the same tactic as sault uses. What does it matter who the messenger is? If you were in a theatre & someone stood up & yelled fire. Would you refuse to leave, even when everyone with a sense of smell could smell the smoke? Would you yell back, 'Are you a fireman, & is your opinion peer reviewed’? Or would you be prepared to acknowledge there was a strong smell of smoke & yes, there are the flames. Perhaps having paid your entry fees you would close your eyes, hold your nose & sit there & burn.
Reply | Report Abuse | Link to thisThe Australian bureau of meteorology states it has been the hottest since records began 150 years ago, the closest comparable was in 1961.
Reply | Report Abuse | Link to thisOn any measure your reference to dogma is completely misplaced for this forum.
Saying something often enough does not make it true!
Witness today's tornadoes, a relatively new and increasing phenomenon in Australia due to increased heat energy available for super cells.
The Australian BOM site is notorious for its alarmism & inaccurate forecasts as I proved on an earlier post. They also distort the data by selective choice of available data. I suggest you take a look at the following link & read the comments: http://blogs.scientificamerican.com/plugged-in/2013/01/08/australias-climate-bureau-get-used-to-record-breaking-heat/
Reply | Report Abuse | Link to this@Geoff Gaherty, I have to question one point in your interesting article. You state: "One of the differences between astronomy and astrology is that astronomy uses the current positions of the sun and planets in the sky, while astrology uses historical positions which have been out-of-date for thousands of years."
Reply | Report Abuse | Link to thisThis is a misconception propagated among a few astronomers who misunderstand the basics of astrology. For over two thousand years, astrologers and astronomers have defined the constellations of the zodiac and the signs of the zodiac as separate entities. The signs are measured by dividing the ecliptic into twelve 30° zones starting with 0° Aries (Vernal Equinox Point). This ecliptic measuring system is used for positioning the Sun, Moon and the planets in celestial latitude and longitude (North and South of the ecliptic). The equatorial system is best used for measuring the position of stars and other objects that are not around the ecliptic. So astronomers and astrologers both use the Vernal Equinox Point or 0° Aries as a base point to locate the past, current and future positions of the Sun, Moon and the planets in our solar system with a high level of precision - even though this point corresponds to a point within the IAU defined constellation of Pisces. In fact, nowadays some astronomers are using software written by astrologers and rely on planetary ephemerides published by astrologers.
There are plenty of criticisms of the practice of astrology, but the precision of their measuring system and any confusion between constellations of the zodiac and the signs of the zodiac are not among them.