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Last month, I took this blog daily and gave it the tag line “Blogging one sidereal day at a time.” Everyday, as I read and explore astronomy, science and academics, I attempt to throw something on this little site and live up to that tagline. Today I launched a second, brand new shiny Earth and Sky blog, I’m going to work to bring them, at least once a week, the best of what I find in my daily astronomy-related explorations (which I will also bring to you). This translates into a new tag line: Exploring the sky, one sidereal day at a time.

One of my silly little disappointments with Star Stryder is that no one has asked me what sidereal day means. This could just reflect how skilled everyone has gotten at googling answers, or perhaps Wikipedia has made us all just a bit wiser with its big collective brain. Whatever the reason, I feel like a little kid running around saying, “I learned all about platypuses today,” only to be asked, “What did you learn about the Louisiana Purchase?”
You know, I don’t want to talk about the Louisiana Purchase. I want to talk about platypuses.

Well, actually, I’d like to talk about sidereal days.
If you are given to watching the sunset and wishing on the first star (which may be a planet) you see each night, you may notice that as the constellations wink into visibility they are in a slightly different place each evening twilight. This change occurs for two reasons: sunset occurs at a different time each night, and the alignment between the Sun, Earth and stars is also a little different each night.

I’m going to wait to discuss this first, sunset-time changing problem, until next week, and just take on this change of alignment, sidereal day causing, problem for now.

Let’s start by imagining December 12. On this cold (northern) winter’s day, a person watching the horizon’s of sunrise and sunset would notice the eastern edge of the constellation Ophiuchus just rising in the east at sunrise and the western edge of Ophiuchus just setting in the west at sunset. This means that on December 12, the Sun is planted between us and the often ignored constellation of the Serpent Bearer. If you’re not particularly fond of snakes, this probably sounds like a good place for the Sun to be.

Now consider what the sky will look like 6 months later on a languid day in June – say on June 12 – and look again through our horizon watcher’s eyes. On this probably hot and hazy day, the bull Taurus will appear to poke above the horizon at sunrise and will follow the Sun below the horizon in the evening. Thus, in June, the Sun is placing itself between us and the Bull.

What is actually changing between these two circumstances is how the Earth, Sun and stars are aligned. Let’s extend our constellations as living breathing heros and monsters analogy a bit, and imagine the 12 commonly known zodiacal signs + Ophiuchus arrayed menacingly around the edge of a giant stadium. Now imagine we are huddled together in the center with a very fat yellow rodeo clown. While most rodeo clowns are quick and nimble, this particular clown is actually just a big plastic blowup clown who is nailed to the center of our arena. As we stand, arms length away from our plastic clown, in one location we can see Ophiuchus leering at us over the clown’s shoulders. If we turn our back on the Clown we now find ourself face to face with the red-eye’d Taurus. If we move 180 degrees around the clown, in a mad clockwise dash, we’ll find ourselves peering over the clown’s shoulder at Taurus, but now our back is left exposed to Ophiuchus, and when we put our back to the clown we are face to face with a hero and his snake.

Just as the monster and hero the clown blocked from us varied with our position, the constellation the Sun appears within depends on where our planet is located within its orbit. Each day we move one giant, frantic step clockwise, as the Sun appears to step from Sagittarius to Capricorn to Aquarius as the months move from January to February to March.

So now you know the story of how the Sun moves through the stars. But I haven’t exactly touched on “sidereal day.”

In our day-to-day life we measure our moments relative to the Sun. From midnight-to-midnight we measure one day in a way that is equal to the 24 hour average length between sun-high noon and sun-high noon. But, those of us who study the stars are in need of a different measure of time, one that instead measures the span of time from star-high moment to star-high moment.

The 24 hour day gives us just enough time for the Earth to start out with your nose facing the Sun, and to get rotated through more then 360 degrees as the Earth steps clockwise around the Sun and rotates back to placing you nose-to-Sun once again.

With the stars, not so much rotation is required. They stay in place, and a 360 degree rotation is all that is ever needed.

Consider the star Caph in Cassiopeia. This bright star marks the end of the short arm of the W that is named after a queen. This star lies right off the zero hour line that marks the beginning of time on the sky. If I measure the minutes from Caph passing through the top of the sky – crossing the meridian – to its return to that same high-sky position, I’ll count 23 hours 56 minutes and 4.1 seconds. This span of time is called one sidereal day. The slight differences between a solar measured day and a sidereal day comes from the extra distance the Earth must rotate to point itself back at the Sun from its new position.

Looking at my sidereal time keeping clock, I know that 9 minutes into a new day, Caph passes through its highest position in the sky. I know that Betelgeuse will always pass through its personal high point at 5:55 into the day. Every sidereal day, the stars will always find themselves arrayed in the same way and the same sidereal moment.

It is only the Sun that forces our view to change. So one solar day at a time I live my life, in our Sun driven society, while the stars make their way one sidereal day at a time through our skies.

So, for anyone who cares to ask, when I say I’m blogging one sidereal day at a time, it means my Star Stryder sidereal-daily blog will average 366.24 posts a year instead of the solar days average 365.24 posts per year.