The varmint was right!

On February 2, Punxsutawney Phil (the groundhog) predicted six more weeks of winter. And, by gum, he was right! It's now six-and-a-half weeks later, and spring has sprung (at least in the northern hemisphere). Today the sun will appear to move north of the equator, where it will stay for the next six months. At the north pole, the sun is rising, and at the south pole, the sun is set (stranding scientists at the South Pole Station until their spring comes).

Tomorrow is the full moon. In the Christian tradition, the first Sunday following the first full moon of spring is the Easter holiday (which explains why Easter is so early this year -- the full moon comes only a day after the start of spring). The Easter holiday remains one of the western calendar's few remaining links to lunar calendars (calendars based on the moon's 29-day cycle through its phases). Lunar calendars are still in popular use throughout the world -- the Islamic calendar, Jewish calendar, and Chinese calendar (among numerous others) are all based primarily on the moon's phase. But since the moon's phases don't match up exactly with the solar year (the length of time it takes the Earth to orbit the sun once, and the basis of the Gregorian calendar we use in everyday life), lunar calendars are a bit odd to the uninitiated, requiring the occasional extra month or something similar to bring the two back in line.

Astronomer's observations are often based on the lunar cycle. The full moon is very bright -- you will notice that you can see fewer stars in the sky when the moon is almost full as opposed to when the moon is absent from the sky. The faintest stars are hidden by the moon's glare. Even with telescopes, the moon's glare hides fainter objects. So, when we ask for telescope time, we have to specify if we can do our science when the moon is bright.

When the moon is bright, astronomers tend to look at bright stars, because their light cuts through the moon's glare. More recently, astronomers also tend to use the full moon time to look at objects in infrared light. Although the moon is still very bright in the infrared, the sky itself is always glowing brightly in the infrared, and the moon's light doesn't add much to the overall glow. (There are also stories of infrared astronomers moving the telescope to their next target and finding the moon is in the way!) But those of us who just want to study faint stars or galaxies in visible light are out of luck near the full moon. So, even as the modern world steers further away from lunar calendars, observational astronomers still have to pay close attention to the phases of the moon.

Thats right, woodchuck-chuckers - it's GROUNDHOG DAY!

Image Source: Melissa Farlow, National Geographic

Well, okay, Groundhog Day is tomorrow. But, as I won't be blogging tomorrow, I wanted to get a word in edgewise on this most noblest of astronomical holidays.

For those of you who may live in a country not blessed with such a glorious holiday, let me explain the day. On February 2, the legendary groundhog, a furry woodland animal that looks exceptionally cute but is not always the most friendly of the beasts, awakens from its winter hibernation and sticks its head out of the ground. If the groundhog sees its shadow, it is spooked (not being among the most clever creatures) and runs back in its hole to sleep for another six weeks. If it doesn't see its shadow, it happily scampers about, and we get an early spring.

Of course, this legend has little to do with reality. The fact is that, whether or not the groundhog sees its shadow, there are another six weeks of winter. Look at your calendar, count 6 1/2 weeks into the future, and you'll see the first day of spring. Now count backward 6 1/2 weeks, and you'll see the first day of winter. Groundhog Day is halfway between the winter solstice and the spring equinox. And that is not coincidence!

These days halfway between an equinox and a solstice are called the cross-quarter days. They used to play a large role in pagan cultures (and still do among neo-pagan religions). We even have "holidays" on or near three of the four cross-quarter days: Groundhog Day, May Day, and Halloween.

What does a cross-quarter day mean for us? Not too much -- most of the northern hemisphere is in the middle of a cold winter, hoping that spring will come soon, and the southern hemisphere is enjoying summer vacation, and hoping that summer lasts a little longer. Around now you will probably start to notice the days getting noticeably longer pretty quickly. Around the winter solstice, the length of the day stays pretty constant for a while, but as the Earth continues to speed around our sun, the sun will appear to rise higher in the sky (for those of us living in the north), the days will get longer, and, eventually, the temperatures will get a bit warmer.

I make a big deal out of days like this, I realize. It's because the seasons are one of the most obvious links we have to the larger Universe around us, and one that most Americans don't understand. And it's a hard picture to grasp -- we are on a tiny ball of a planet that spins once a day and goes in circles around our parent sun once a year, which itself is orbiting the Milky Way galaxy every 250 million years, and our galaxy which is moving around our neighbor galaxies and slowly falling toward a cluster of galaxies 60 million light-years away. We're in constant motion, most of it extraordinarily slow by galactic standards. But we can sense some of it -- our planet's spinning, and its motion around the sun -- and these motions intimately affect our lives. Once we can grasp that, we've started to open our minds to a much larger universe around us.

Lunar Calendars

This week is a week of holy days for people of the Jewish and Christian faiths. For Judaism, the Passover celebration began last night; for Christians (both western and eastern), Easter is this Sunday. The exact days change from year to year. Why?

The Jewish calendar is based to a large extent on the moon. The moon completes a cycle of phases every 29.5 days; twelve "moonths" (or lunar months) add up to a bit over 354 days. Note that this is 11 days short of a normal, or "solar" year. So, the Passover celebration, which occurs in the Jewish religious month of Nissan, can shift with regard to the Gregorian calendar that we all use. Every few years, an extra month has to be added to the calendar to keep the lunar calendar in line with the sun, so that the Passover celebration always occurs in the spring.

The date of Easter used to be fixed to the date of Passover, since the Christian holiday is based on events that occurred during the Passover celebration. More recently, the date was changed so that Easter is the first Sunday following the first full moon of spring. In western Christianity, most years the two holidays coincide. In eastern (orthodox) Christianity, due to vagaries of different calendars used, the Easter celebration is often a lunar cycle later (though this year they coincide).

There are many other calendars based on the lunar cycle. The Islamic calendar sticks very precisely to the lunar cycle, so dates of Islamic holidays drift with respect to the seasons. Most other lunar calendars have an extra month added in now and then to try and keep track with the seasons.

Calendars are all, to some extent, artificial. There are not an even number of days in a lunar cycle (or in a year, which is why we need leap days now and then), there are not an equal number of lunar cycles in a year. But for the purposes of time keeping, a day, a lunar cycle, and a year are natural divisions of time if you don't have watches and calendars. In fact, in many ways the Gregorian calendar that we use is even more arbitrary -- a month doesn't follow the lunar cycle, and the lengths of the months vary from 28 to 31 days in an irregular fashion, and every 4th year (except every 100th year, except once again for every 400th year) we add an extra day to our calendar.

If you are celebrating holy days this week, a happy Passover or happy Easter to you all. And, even if you aren't celebrating, you know that the full moon is this week, so go out in the evening and enjoy the big yellow moon.