Z is for Zodiac

This isn't about telling your horoscope, but knowing where the Sun is at any particular time of the year. (Those born under the sign of Ophiuchus the Snake-bearer know that Astrology is not a science because nobody has been able to prove its reasoning through the scientific method.)

In Astronomy, there are thirteen constellations that reside along the ecliptic (the path the Sun takes through the sky year-round). This means as the Sun moves through the sky from month to month, it will be found within the boundaries of certain constellations.

Astrologically, the sky is divided into twelve "houses" of 30° each. In astronomy, the constellations of the astronomical zodiac is not so evenly divided, and we've thrown in one more constellation because the Sun does spend some time in one corner of it. The planets and the Moon also move within the ecliptic.

Because the Earth is tilted, the ecliptic doesn't match up with the equator except on equinoxes twice a year.

The red line is the path the Sun takes. The green line is the equator.

When it comes to measuring where stuff is in the sky, there's two axes of celestial coordinates: declination (DEC) and right ascension (RA).

Declination measures north/south in degrees: North (90°) to Equator (0°) to South (-90°).
Right ascension measures eastward from a point of origin (the vernal/spring equinox) in hours, with there being 24 hours in a full circle, due to the rotation of the Earth. This is because astronomers measure right ascension by timing when an object passes through the highest point in the sky, or the meridian. Each hour is about 15° in width.

Anything anywhere in the sky can be given a set of coordinates.

Looking at the star of Betelgeuse:  DEC +07° 24′, RA 05h 55m

This means Betelgeuse sits about seven degrees north of the equator, and on the spring equinox (21 March), it takes about five hours and fifty-five minutes before it reaches the meridian of the sky.

Let's look at the Zodiac astronomically.

Here's the actual map:

As you can see, the constellations take up different areas of real estate. Sometimes the Sun will spend as little as a few days in some constellations and several weeks in others.

Right ascension starts on the spring equinox and is also called the "First Point in Aries"... however, due to precession, the spring equinox actually lies in Pisces today!  Here's how much it's shifted over the past seven thousand years:

(A brief word about something called precession: the Earth wobbles on a long-term cycle of about 26,000 years, where her north pole points to different parts of the sky. (Polaris isn't always going to be the North Star.) Because of this, the constellations have shifted from where they were originally observed a few thousand years ago, and don't line up with the calendar we know and love today. Your astrological zodiac sign no longer corresponds with the constellation of the same name. Sorry.)

We'll start on the vernal equinox and have a look at all the Zodiac constellations.

Constellation:	Coordinates:	Sun enters/exits:	Time in constellation:
Pisces	RA: 1h DEC: 5°	12 March – 18 April	38 days
Aries	RA: 2h DEC: 15°	19 April - 13 May	25 days
Taurus	RA: 4h DEC: 15°	14 May - 19 June	37 days
Gemini	RA: 7h DEC: 20°	20 June - 20 July	31 days
Cancer	RA: 9h DEC: 20°	21 July - 9 Aug	20 days
Leo	RA: 11h DEC: 15°	10 Aug - 15 Sept	37 days
Virgo	RA:13h DEC:0°	16 Sept - 30 Oct	45 days
Libra	RA: 15h DEC: -15°	31 Oct - 22 Nov	23 days
Scorpio	RA: 17h DEC: -30°	23 Nov - 29 Nov	7 days
Ophiuchus	RA: 17h DEC:-30°	30 Nov - 17 Dec	18 days
Sagittarius	RA: 19h DEC: -25°	18 Dec - 18 Jan	32 days
Capricorn	RA: 21h DEC: -20°	19 Jan - 15 Feb	28 days
Aquarius	RA: 22h DEC: -10°	16 Feb - 11 March	24 days

Which is your favourite zodiac constellation?

What is your astrological zodiac sign, and what is your astronomical zodiac constellation (based on your birthday)?

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Her Grace is fond of Scorpius, because it really does look like a scorpion.

If you wish to explore more Astrological applications of the Zodiac, check out Chris Votey's "Madness of a Modern writer" A to Z challenge where he's been combining the Greek and Chinese Zodiacs to create character profiles.  It's been fun for me, from a writer's point of view.

Y is for Year

Happy Birthday to you, if you happen to have a birthday this year. (Sorry, Leap Year Babies. No birthday cake for you.)

Essentially, a year is the time it takes for the Earth to go around the Sun. So yes, Venus has a Venusian year (0.6a) and Mars has a Martian year (1.88a). But for the purpose of today, I'm going to talk about how the Year is a standard unit of measurement.

Astronomers need a way of measuring things. Since there's no standard galactic measuring stick for, well, everything, we've taken what's most familiar and made that our basis. For example, the mass of planets is measured by the mass of the Earth (M_⊕) and the mass of stars is measured by the mass of the Sun (M_☉). Short distances are measured by Astronomical Units (AU), which is the mean distance from the Earth to the Sun and long distances are measured by lightyears (ly) (the distance it takes for light to travel a year).

In astronomy, one measurement of time is the Julian year (symbol: a), which is exactly 86,400 seconds (as seconds are the base unit of time in SI). This equates to about 365.25 days, if that makes your brain hurt less. That's a very familiar number, with our calendar years being 365 days, except for every four years, when we add up the .25 of a day, and tack on an extra Leap day, so our days can sync up with our years. Our current Gregorian Calendar is based off this cycle.

While we've known about this extra quarter-day for a few thousand years, we didn't realise exactly how precise we'd not calculated it, so our earlier calendars had a bit of drift going on, and occasionally needed serious correction. That's why the ten days of Oct 5-15 1582 AD (CE) don't actually exist. Also why Ramadan appears to drift in relation to our civil calendars. And if you were born in Sweden in February 30, 1712, I am very, very sorry for you. Here's three hundred years' worth of birthday cake to make up for that double-leap day.

Let's put calendars aside and talk astronomy.

Julian years are used to measure duration. For example, how long would it take light to reach us from Alpha Centauri? About 4.6 years. (A Julian year is what they use to calculate a lightyear.)

How long does it take for Jupiter to go around the Sun (aka a Jovian year)?  11.8618 (Julian) years.

There's other types of years such as the sidereal, tropical and draconic years, used to measure stuff in relation to Earth but for general astronomical purposes in measuring duration in the rest of the Universe, we prefer the Julian year. Feel free to go hardcore if you wish regarding the other year types.

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Her Grace does not mind collecting years as she goes along. Old age is a privilege denied to many.

X is for X-ray

Finally, an AtoZ blog entry where I don't have to stretch to find an entry.

We've talked a bit about the electromagnetic spectrum during the A to Z--Radio, Infrared, Visible (Optical) and Ultraviolet--because light is primarily the only tool we have to explore the universe.

While other bloggers are really pushing it to find something for the letter X, here's something that comes naturally to astronomers: X-rays

We all know about X-rays for their medical uses: broken a bone or been to the dentist, chances are you had an X-ray photograph taken.

X-rays are cool because they can penetrate certain types of matter and show us other types. This is because they're highly energetic. Naturally, this is a good thing for astronomers. We like looking at high-energy things.

X-rays were discovered in 1895 by Wilhelm Röntgen. Here's his original paper: German,  English Translation

"Hand mit ringen," Will said.
His wife said, "I have seen my death!"  Drama queen.

In fact, X-rays are often called Röntgen rays. What did Wilhelm Röntgen call them?  X-rays, with X standing for "mysterious", because he really wasn't sure what they were at first. Eventually he and a few scientists figured it out.  A few early articles about Röntgen's mysterious X-rays.

And, like any other scientist that thought science stuff was nifty, he played around with it, and even freaked his wife out by using her hand as a guinea pig by taking the first X-ray photograph.

This was fascinating, as it's really the image of an X-ray shadow, as the minerals of the bones block out the X-rays. Lead is also good for blocking out X-rays.

So, what makes X-rays so useful for astronomy?

Remember how stars come in different colours, depending on how hot they are?  If you boost the temperature of an object in outer space to waaay hot (more than a million Kelvin), its peak colour goes on beyond blue all the way up into X-rays.

This makes X-rays really useful for detecting high-energy events and objects.  Neutron stars and accreting black holes emit X-rays. Supernovae emit X-rays. (Stars emit X-rays, as they emit through the whole EM spectrum, though not to the same degree the Really Powerful Stuff, like active galactic nuclei, does.) Want to know where all the moving and shaking is happening in the universe?  Look for the bright X-ray spots.

Now, X-rays are absorbed by our atmosphere (thankfully), so any X-ray observatories need to be in orbit, like the Chandra X-Ray Observatory.

Here's some cool pictures taken in X-ray:

Looks very different from the hand mit ringen, as these are not images of the shadows of X-rays, but rather the emissions of X-rays. That's why they're so bright.

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Her Grace once calculated how much X-rays she emitted. Answer: not much.

W is for Wobble

Lots of things wobble. This is due to gravitational influence by something else.

Mass has gravity, and gravity is a very social thing. Two bits of mass spy each other across the room (or the universe) and try to get closer. If they get close enough, they end up in an orbit around each other, possibly in a spiral until they collapse together, though this doesn't always happen if the escape velocity is sufficient to keep them from falling into each other.  (That's what most orbits are: falling, but failing to hit the ground.)

The centre of gravity about which these object spiral about is called the barycenter.

Looks like this:

Do-si-do.  The barycenter is the red cross in the middle.

When the objects are of a similar size, the barycenter will be somewhere in between the two objects, as is the case with Pluto and Charon:

Pluto and Charon and their barycenter, the cross in the middle.

Pluto has a small orbit around the barycenter while Charon has a larger one. So they kind of wobble like an uneven barbell.  Because the barycenter is outside of Pluto, some people would like to call Pluto/Charon a double planet, rather than a planet and a satellite. I'm cool with that.

When one object is distinctly more massive than the other, the barycenter will be located within the larger object, such as the Earth and the Moon:

This gives the Earth a little bit of a wobble, instead of a full-on do-si-do.

We noticed our Sun has a bit of a wobble, due to the gravitational influence of the planets about it. In fact, the Sun's wobbled quite a bit:

Notice how sometimes the barycenter's inside the sun and sometimes its not?

Yeah, not surprising, considering what we know about gravity. Then some clever soul thought, if our Sun wobbles due to planets pulling on it, wouldn't other stars wobble for the same reason?

Whoa! Mind blown! So we started looking at nearby stars with this Wobble Method (also called Radial Velocity or Doppler Spectroscopy Methods) to see if they had a wobble.

Ohmigosh, they did! Thus, we discovered exoplanets.  And we said, "That's so cool!!!" (Then we felt slightly stupid because we hadn't figured something so simple out until now.)

Gamma Cephei Ab was the first exoplanet detected in 1989 (confirmed in 2002), and since then, we've uncovered evidence for thousands of exoplanets.

Nice to know we're not alone.

Now, there's lots of other methods for detecting exoplanets, but it all started with noticing how gravity made things wobble.

Wobbling isn't just for detecting exoplanets. Lots of other discoveries are due to the observation of wobbliness.  Go hardcore and check out wobbliness from asteroids with moons to entire galaxy clusters.  Even browsing through the list of titles is fascinating. Never be afraid to read an abstract. Save your freaking out for the paper itself.

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Her Grace believes that love makes the world go round, but gravity rules the universe.

V is for Visible Spectrum

Until the past century, pretty much all astronomical work took place with the observance of the visible spectrum. MK-1 Eyeball (the naked eye) was how observation took place.

The visible spectrum isn't terribly big--only 390 to 700 nm--considering just how wide the electromagnetic spectrum is. Yet within that short range, we've been able to accomplish some magnificent astronomy.

Before we discovered calorific rays (infrared) or chemical rays (ultraviolet), we were playing with the visible spectrum. Spectroscopy allowed us to divide up light into its different wavelengths and observe the universe. You can tell a lot about the composition of something by the frequency and amplitude of the light it emits.

Early scientists (like Herschel), noticed that certain spectra had dark lines. These are absorption lines, when atoms (like hydrogen) absorb certain light frequency.

Johann Balmer noticed that hydrogen absorbed certain frequencies. This helped us discover just how much hydrogen was out there, and also determine the nature of the hydrogen atom itself. (Hardcore: the absorption lines happen when a hydrogen electron absorbs that photon's energy, thus causing it to jump or transition to the next electron shell.)

All elements exhibit absorption lines. While some are seen only "off-stage" (ie, in the infrared or ultrviolet or beyond, especially if red-shifted), many can be seen in the visible spectrum.

Here's the visible spectrum of the Sun. As you can see, there's plenty of dirty metals lurking within our nearest and dearest star. (Can you find sodium?  Hint: it's orange.)

Click here to embiggen.

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Her Grace is best viewed in the visible spectrum.

U is for Ultraviolet

Yesterday we talked about lightbuckets and the light we can capture in them. Today we'll talk about one of those frequencies, Ultraviolet.  Most people know about ultraviolet from sunscreen commercials, and good old Slip Slop Slap campaigns.

In 1800 Herschel discovered Infrared (he called them Caloric Rays because they felt warm). Like any good scientist, he wondered if something similar could be found on the violet end of the spectrum. Alas, when he set up his thermometer on beyond violet, he didn't detect any heat. Oh well, he said, and focused on what he knew.

Then in 1801 Johann Wilhelm Ritter refused to be daunted by the violet end of the spectrum. See, he'd read Herschel's paper, and thought that Herschel hadn't gone far enough in his investigations. As a chemist he was familiar with the photosensitivity of certain chemicals. He knew that silver chloride turned dark when exposed to sunlight, reacting stronger to blue light, rather than red.

When he exposed a piece of paper coated with silver chloride to a spectrum, he discovered the strongest reaction was in the “invisible” side of violet light. Because of the chemical reaction, he named this invisible band of light “deoxidizing rays” or “chemical rays”. Essentially, he was one of the first to create a "photo-graph", or a literal recording of light.

He'd discovered Ultraviolet.

But what he also discovered was ionising radiation.

The electromagnetic band is divided into two different kinds of radiation: non-ionising waves, like radio waves, and ionising radiation, like X-rays.

Ionising radiation is radiation that's strong enough to knock electrons off an atom or molecule. The ability to cause a reaction in a chemical such as silver chloride is due to ionisation. It's the scary stuff that can potentially damage DNA and cause cancer.

Fortunately, most of the harmful ionising radiation (from UV on up to gamma rays) gets filtered out by our atmosphere. Still, visible light and some near UV get through. These are the frequencies that start to chemically affect stuff. (This is also why you don't store your beer in sunlight.)

Non-ionising radiation like radio waves can pass through our atmosphere, but doesn't pose a threat to us. (FYI, the radio wavelengths that mobile phones use is safely in the radio waves range. Mobile Phones do not cause cancer. Feel free to carry them in your bra.)

Ah, so what's the benefit of the ultraviolet spectrum in astronomy? It's good for detecting hotter objects. UV is very good for detecting chemical composition, very old stars or very young stars, and for identifying star-forming regions, which denotes active galaxies.

This UV images of the Andromeda Galaxy (M31) shows it's active star-forming areas:

When the variable star Mira was imaged in UV by GALEX in 2006, scientists were amazed to discover it had a tail. Mira moves through space rather quickly for a star (130km/s), so fast, it even has a bit of bow shock in the interstellar medium (ISM) and a tail of matter streaming behind it for thirteen lightyears:

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Her Grace likes to think of UV as the "hot and bothered" frequency.

T is for Telescope

The Universe is full of light. Therefore, if we want to know more about the Universe, we need to sample some of that light for analysis. One of the best ways to capture that light is in a lightbucket called a telescope.

For thousands of years astronomers only had one method of capturing light--MK-1 Eyeball, aka the naked eye. Look up in the sky, what do you see?

Lots of stars, most of the major planets and a few other fuzzy objects were easily observable with the naked eye. It had its limitations, as it only detected wavelength between 390 to 700 nm. Also, it was limited in its resolution and the number of photons it could capture.

For a few thousand years humans knew that carving glass into certain shapes could bend light, focusing it, bringing more photons to the human eyeball. Then really recently, in the early 1600's, a few bright sparks put a couple of lenses in a tube, looked through and yelped, "Wow! I can see far! Tele-scope!" The first refracting telescope was born. Thomas Harriot thought it would be nifty to look at the sky through this thing. He was right. To him, the Moon looked awesome. He could see such detail!

Galileo and his refractor.

About the same time, Galileo built his own and looked upwards. To his amazement, he found four moons orbiting Jupiter. These details were not visible previously. The ability of the telescope to capture more photos and resolve very distant objects was totally amazing.

Newton's reflecting telescope.

Humans are lots of fun because they'll take an idea and run with it, seeing if they can improve on the original design. Another bright spark (some guy named Newton) wondered if a parabolic mirror could serve just as well for focusing the light. Sure enough, it worked wonders.

So lots of gentleman scientists played with this new tele-scope technology, improving it in size and quality, and peered into the heavens with it. At first, it was mostly planets they stared at, and various nebulae, as the stars were too far away to resolve to anything but points of light.

That didn't stop them from having fun with the light they captured. Prisms were notorious for breaking plain light up into pretty rainbows called spectra (singular: spectrum). When that happened, they then discovered things like absorption lines, infrared,  and ultraviolet.

You can go really big with radio telescopes,
like they did in Arecibo.

The infrared and ultraviolet discoveries really sparked some imagination. Could there really be "invisible" light beyond the visible spectrum? If so, could we capture it?

Sure.

With lower frequencies such as radio waves, they discovered they could be captured with antennas. Later, radio dishes (very similar in shape to the parabolic mirrors used to capture light) helped focus radio waves onto the receiving antenna, instead of just trying to pick up any old radio wave that happened to bounce by, like the aerials on our rooftops.

X-ray telescopes, same thing. A large parabolic mirror focuses X-ray wavelengths onto an X-ray detector. Problem with X-ray telescopes is that they're rather useless on Earth, as our atmosphere blocks out most stellar X-rays. So if we wanna gather X-rays, we've got to put our light buckets into orbit.

X-ray telescopes, like the Athena X-ray Observatory, can have lots of fun capturing the emissions of X-ray sources from orbit.

By the end of the 20th Century, humankind had come up with all kinds of telescopes to observe different kinds of electromagnetism. We're very good at capturing photons of all wavelengths, studying them, and thus, through sheer observation alone, we know about our Universe.

Do you have a telescope? If not, have you had a chance to look through one? If you haven't, see if you can find a local star party. Many planetariums and astronomical clubs hold them regularly. I recommend waiting a few months for Saturn to rise, for that is one spectacular planet to look at through a telescope.


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Her Grace observes through a Celestron Nexstar 130SLT 5" reflector.