6degrees AstroBlog

In the spring a young man’s fancy lightly turns to thoughts of love

By Irwin Horowitz, 3-01-08

	Seasons in Northern Hemisphere

Astronomy impacts our everyday lives in ways most people rarely consider.  For example, our methods of recording time on a daily, monthly or annual basis have their origins in various astronomical cycles.  Similarly, we recognize the different seasons on our calendars but often don’t understand their astronomical source.

March heralds the passage of winter into spring in the northern hemisphere and of summer into autumn for locales south of the equator.  But why is this so?  Why do our “days” grow longer?

Contrary to the beliefs of some individuals, the distance between the Earth and Sun has no significant impact on the seasons.  In fact, we are actually closest to the Sun in January, during the depths of winter in the north.  In contrast, we are furthest from the Sun in July, when it is warmest in our part of the world.  The variation in distance is not significant enough to impact our weather in any measurable way.

Our planet orbits around the Sun in a plane known as the ecliptic.  The origin of this term is related to the occurrence of eclipses when the Sun, Earth and Moon all lie in this plane.  The rotation axis of the Earth is tilted relative to the ecliptic plane by an angle of about 23.5 degrees.

To visualize this, imagine a sheet of paper which represents the plane of the ecliptic.  A pencil punched through the sheet represents the spin axis of the Earth.  If the pencil was perpendicular to the paper, the Earth would rotate in a manner such that the Sun would always be above the equator and there would be no seasons.  By tilting the pencil about ¼ of the way down, you can observe the angle which the Earth is inclined relative to the ecliptic.

As we orbit about the Sun, this tilt remains nearly constant in space, so as a result, there are times during the year when the northern hemisphere is most tilted towards the Sun (summer solstice) and when it is most tilted away from the Sun (winter solstice).  In between these two extremes, there are also two moments each year when the Sun appears directly over the equator.  These are known as the vernal (spring) and autumnal equinoxes.

Since the vernal equinox occurs around March 20 each year, this is when spring officially starts in this hemisphere.  It occurs at the moment the midpoint of the Sun appears to cross the plane formed by extending the Earth’s equator out into space, as the Sun heads from the southern hemisphere to the northern hemisphere.  This year, that moment is on 20 Mar at 0548 GMT (or 19 Mar at 11:48 p.m. MDT).

The vernal equinox is also of importance to astronomers as the reference point for the most commonly used coordinate system on the sky.  This is known as the equatorial coordinate system and consists of two separate components.  The first, right ascension is analogous to longitude here on Earth.  It is measured from west to east and quantified using units of time (hours, minutes and seconds).  There are twenty-four hours of right ascension around the entire sky.

The second component is called declination and is closely tied to the latitude system here on Earth.  Declination is measured north (+) or south (-) of the equator using units of degrees from zero to ninety.  The celestial equator is at zero degrees declination.  The north celestial pole (near the North Star) is at +90 degrees declination, while the south celestial pole is at -90 degrees.

Using this system of coordinates, astronomers can identify the location of any object in the sky from any set location on Earth.  If you sometimes wonder how we can find various astronomical objects of interest using our telescopes, when there appears to be nothing visible to your eyes, this coordinate system is one of the most useful means of accomplishing this feat.

But what does any of this have to do with the vernal equinox?  The starting point for right ascension is completely arbitrary (unlike the celestial equator for declination).  Where do astronomers choose to define zero hours of right ascension?  It is defined to coincide with the location of the Sun at the vernal equinox.

In ancient times, this was located on the border of the constellation Aries, and was known as the “first point in Aries.” However, due to the effects of precession, the vernal equinox has moved relative to the distant stars so now it is located in the constellation of Pisces and in a few centuries it will venture into Aquarius (“the Age of Aquarius” for all the “Hair” fans out there!).

So, what is this phenomenon of precession?  Remember the pencil through the sheet of paper?  Imagine that over time, you slowly rotate the pencil point around an imaginary axis perpendicular to the paper.  It behaves much like a gyroscope or spinning top by “precessing” about this axis.  However, it takes nearly 26,000 years to make a single rotation!

Therefore, in about 13,000 years, the seasons would be completely flipped from what we know today.  In order to avoid celebrating the vernal equinox in September we include leap days every few years to make sure our calendar remains roughly aligned with the equinoxes and solstices.

Culturally, there are numerous examples of how the vernal equinox impacts society.  Many religious traditions look to this moment to define their rites and celebrations.  Best known in western societies is the timing of the Christian celebration of Easter.  As all Christians know, the annual date of Easter is not fixed to the Gregorian calendar.  Instead, it is based on a combination of both the solar cycle and the lunar cycle.  It occurs on the first Sunday following the first Full Moon that occurs after the “vernal equinox” (which is set to 21 Mar for these purposes).

This year, since the Full Moon occurs on 21 Mar, Easter will be celebrated the following Sunday, 23 Mar.  This is one of the earliest dates possible for the celebration of this holiday.

Ancient societies, such as the Druids in England and the Maya in Central America, built huge monuments to track the motion of both the Sun and the Moon.  This knowledge was critical so that these societies would know when to plant and harvest their crops and when to engage in various religious rituals and ceremonies.

The most famous of these monuments is Stonehenge, near Salisbury, England.  Today we conjecture that the placement of the various monolithic stones was intended to observe the rising and setting locations of the Sun at both the solstices as well as the equinoxes.  In this manner, the ancient society responsible for the construction of Stonehenge could use it as a form of calendar.
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The sky in March continues to be dominated by both Mars and Saturn in the early evenings.  Mars is located in Gemini, near the foot of Castor.  In the two months since opposition, Mars has diminished in size by nearly a factor of two.  It will continue to get smaller and fainter as we pull away from it as we orbit the Sun.  It will pass close to the open cluster M35 this month around the 9th and 10th, making a lovely site through a pair of binoculars.

Saturn is in Leo, slightly east of the bright star Regulus.  Last month, during the total lunar eclipse on 20 Feb, it was possible to view both Saturn and Regulus corralling the Moon between them.

Jupiter will rise in the early morning and be easily visible before sunrise low in the southeast.  The remaining planets are too close to the Sun to be easily visible this month.

Here in the United States, we will begin to observe daylight saving time on the morning of 09 Mar.  This means you should remember to set your clocks ahead one hour before going to sleep next Saturday.

The Boise Astronomical Society will hold their monthly membership meeting on Friday, 14 Mar at 7 p.m. in Classroom #2 of the Discovery Center of Idaho.  This month, John McVey, a member of BAS, will give a presentation and demonstration of digital setting circles and the ‘Push-to’ computer, using his behemoth 16” Dobsonian telescope.

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