Our inaugural edition of the Almanack, Vol. I, No. 1, for January, 2012 features the following sky sightings:
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Finding Venus and Jupiter;
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The changing position of the sunset on the horizon;
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Learning the evening constellations;
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See Orion and the winter constellations rising;
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Identify the morning constellations before sunrise;
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Find Mars and Saturn in the morning sky; and
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Follow the retrogradation of Mars.
The Classical Astronomy
Celestial Almanack is 17 pages long and only costs
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2012 promises to be a VERY EXCITING year for astronomy, including:
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A spectacular planetary conjunction of Jupiter and Venus in March;
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The opposition of Mars and its retrogradation in the spring;
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An annular solar eclipse over North America in May;
- The historic Transit of Venus in June; and
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A planetary conjunction of Mars and Saturn with Spica in August.
You'll not want to miss coverage of these exciting sky events. Read the
Celestial Almanack every month to follow all the interesting events over the coming year. And be sure to tell all your friends! Thanks to everyone for your support.
What's Up with Those Seasonal Starting Times?
This year, it announced in the news media that "winter begins at 12:30 AM, eastern standard time, on December 22." If you're like me, you probably wonder why the seasons always begin on different days, and why they always begin at such odd times of the day or night. For example, why is the winter solstice usually on December 21, but sometimes, like in 2011, on December 22?
The short answer is, this happens because the actual solar year, the time between one winter solstice and the next, is not a precise number of days, or even a specific number of hours. The solar year is related to the time it takes the Earth to complete its orbit around the Sun. This is completely separate from the period of Earth's rotation that causes day and night. Also, the length of the year can vary a bit because of minute changes in the Earth's orbit, resulting from the interaction with the gravity of the other planets, so every solar year is not the same precise length. Anyway, that's the short answer. The long answer is... well, long!
Traditionally, since ancient times, winter in the northern hemisphere is reckoned to begin at the precise moment the Sun reaches its most southerly position in the sky, when it is directly overhead at the Tropic of Capricorn. Spring begins when the Sun is centered on the celestial equator, when it is moving north along the ecliptic, and is directly overhead at the Earth's equator. Summer begins when the Sun reaches its northern extreme in the sky, and is overhead at the Tropic of Cancer. Autumn begins when the Sun again crosses the equator, heading south.
Though most people today are not well-versed in Classical Astronomy, everybody knows about leap year. We all know that, every four years, there is a February 29, instead of the usual 28 days in February. We add this extra day to the regular 365 day calendar since the actual length of the solar year is about 365-1/4 days. This keeps the calendar nice and tidy, so that the seasons begin on the same dates.
If we didn't add this extra intercalary day every fourth February, the winter solstice would eventually slip forward through the months. As happened in 2011, the winter solstice landed on December 22. Since 2012 is a leap year, the winter solstice will be back on December 21 for 2012. But if we didn't add February 29, by 2016, the winter solstice would fall on December 23, and by 2020, it would land on Christmas Eve. Before too many more years, the solstice would begin to arrive in January, and over time, all the seasons would begin to slip forward through the months. So adding an extra day every leap year makes sure that the seasons arrive on roughly the same dates over the years.
However, it just so happens that the solar year is not precisely 365 days, 6 hours, but is actually about 11 minutes and 18 seconds shorter than that. The actual length of the solar year is about 365 days, 5 hours, 48 minutes and 42 seconds. So the times of the seasonal changes arrive about 5 hours and 48-1/3 minutes earlier every year compared to the year before. Here's a chart of recent winter solstice times that will hopefully make the idea more clear.
Winter Solstice (eastern standard time)
2008, Dec 21 7:04 AM
2009, Dec 21 12:47 PM
2010, Dec 21 6:38 PM
2011, Dec 22 12:30 AM
2012, Dec 21 6:12 AM.
(The times are not all exactly 11 minutes, 18 seconds earlier than the year before due to tiny gravitational bumps in the Earth's orbit, caused by interactions with the gravity of the other planets. Some solar years are longer by a few seconds, and others are shorter, but it all averages out in the end.)
As you can see from the chart, after four years, from 2008 to 2012, the time of the winter solstice arrives back at the early morning of December 21, but about 52 minutes earlier. Eleven minutes a year may not seem like much, but over decades of time, it begins to add up. Since the solstice times can slip nearly an hour in only four years, the slippage adds up to the better part of a day after a century.
It might seem like such precise timekeeping is the product of modern high-tech science, but it was known in ancient times that the solar year was less than 365-1/4 days. The Greek astronomer Hipparchus, in 200 B.C., calculated that the solar year was short by about 1/300th of a day.
Even in ancient times, it was understood that the shortfall in the solar year was causing the dates of the solstices and the equinoxes to slip backwards over the span of centuries. By the Renaissance period, it was discovered that the error amounted to three days every 400 years. In order to keep the calendar on track with the seasons, the Gregorian Calendar was adopted in A.D. 1582. The proclamation of Pope Gregory XIII established that century years would not be leap years, except once every four hundred years:
Thereafter, so the Equinox does not recede from March 21 in the future, we also establish that a leap year every fourth year must be continued (as is customary), except in century years.... In this way, the years 1700, 1800 and 1900 will not be leap years. The Year 2000 will be a leap year with an intercalated day, February 29, as is our customary habit, and continuously the same order of intermittent intercalated leap year days, every 400 years, will be perpetually maintained. - Gregory XIII
In this way, the seasons were kept on track, so that the solstices and equinoxes would arrive at the same dates. The Gregorian calendar is explained in greater detail in the Epilogue of
Signs & Seasons.
Anyway, the next time you hear a TV weatherman announce that a season is beginning at an odd hour of the day or night, keep in mind that it's just an weird little thing that happens because the length of the year is not a nice, even number of days and hours.
Til next time, God bless and clear skies!
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