Happy September Equinox: An Explanation

 

September EquinoxToday is the September Equinox. You’ve probably already heard it a few times today; people running around proclaiming with utmost exuberance how today is the first day of Fall. In the Northern Hemisphere, the announcement is the harbinger of shorter days and dropping temperatures. But what is really going on today?

When someone says today is the first day of Fall, what they really mean (whether they know it or not) is that today represents an equinox; specifically, the September Equinox.1 On Earth, an equinox is the point in its orbit around the Sun when both hemispheres are equally illuminated; our tilted Earth lines up to a point in which the Sun passes directly over the equator. This happens twice a year, on the March and September equinoctes (that’s the plural form of equinox, use this information smugly).

Contrary to popular belief, the day of the equinox does not represent the day where daylight and darkness are equal. You can thank geometry, the atmosphere, and the Sun’s angular diameter to cause that equality to happen at different times geographically. What today does mean though, is that the equinoctes are the only two days in which the Sun rises due-East and sets due-West, and which the Sun would pass directly overhead from an observer on the equator.

One other very important thing that you must know if you don’t learn anything else today: Way too many people believe that the equinoctes are the only day of the year that an egg can be balanced on its end. While it is true that on the equinox an egg can be balanced, it’s also true of every other day of the year; it makes no difference!

There are other times during the year (read: our orbit around the Sun) that we recognize Earth residing at a special place.  There’s Perihelion and Aphelion, and then the widely-celebrated solstices, but I’ll save those for another time.

Happy September Equinox!

  1. What about them being called the Spring  and Fall (or their Latin names, Vernal and Autumnal) equinoctes? Well, that wasn’t exactly fair to those in the Southern Hemisphere, whose seasons are opposite those in the Northern Hemisphere.

Happy March Equinox: An Explanation

March Equinox graphic

Solstice and equinox diagram, showing the March equinox

Image Credit: NASA

Today is the March Equinox. You’ve probably already heard it a few times today; people running around proclaiming with utmost exuberance how today is the first day of Spring. After the long winters that some of us endure, the arrival of Spring is welcome news. But what is really going on today? After all, where I live it still feels like the middle of Winter, but flowers were already blooming on a trip I took to California a couple of weeks ago1. If we based “The First Day of Spring” on climate patterns, regions across the globe would be recognizing a wide variety of days throughout the year.

When someone says today is the first day of Spring, what they really mean (whether they know it or not) is that today represents an equinox; specifically, the March Equinox.2 On Earth, an equinox is the point in its orbit around the Sun when both hemispheres are equally illuminated; our tilted Earth lines up to a point in which the Sun passes directly over the equator. This happens twice a year, on the March and September equinoctes (which I learned today is the proper plural form of the word equinox).

Contrary to popular belief, the day of the equinox does not represent the day where daylight and darkness are equal. You can thank geometry, the atmosphere, and the Sun’s angular diameter to cause that equality to happen at different times geographically. What today does mean though, is that the equinoctes are the only two days in which the Sun rises due-East and sets due-West, and which the Sun would pass directly overhead from an observer on the equator.

One other very important thing that you must know if you don’t learn anything else today. Way too many people believe that the equinoctes are the only day of the year that an egg can be balanced on its end. While its true that on the equinox an egg can be balanced, it’s also true of every other day of the year; it makes no difference!

There are other times during the year (read: our orbit around the Sun) that we recognize Earth residing at a special place.  There’s Perihelion (which we went over in January) and Aphelion, and then the widely-celebrated solstices; but I’ll save that for another time.

Happy March Equinox!


This article originally posted on March 20, 2012.

  1. In fact, while it may have still been Winter to the San Diegans giving me quizzical looks for swimming in the ocean without a wet suit, to an Alaskan like myself it felt like an unusually warm Summer’s day!
  2. What about them being called the Spring  and Fall (or their Latin names, Vernal and Autumnal) equinoctes? Well, that wasn’t exactly fair to those in the Southern Hemisphere, whose seasons are opposite those in the Northern Hemisphere.

Cosmic Paparazzi: Cat's Eye Nebula

The Cat's Eye Nebula

The Cat’s Eye Nebula – Credit: NASA, ESA, HEIC, and The Hubble Heritage Team (STScI/AURA)

(Click the image for a larger version)

Hubblesite.org: The Cat’s Eye Nebula, one of the first planetary nebulae discovered, also has one of the most complex forms known to this kind of nebula. Eleven rings, or shells, of gas make up the Cat’s Eye.

We recently checked out Supernova 1987A. If you remember, I told you that when massive (8 times the mass of the Sun or greater) stars die they explode in a supernova and leave behind either a neutron star or a black hole, surrounded by a supernova remnant.

When a low mass star (less than 8 times the Sun’s mass) dies, it leaves behind a white dwarf and a planetary nebula. An example of which is shown above, the Cat’s Eye Nebula (also designated NGC 6543). 4 or 5 billion years from now, our own Sun will undergo this very same process.

Perhaps our distant ancestors, or even members of an alien species, will look towards the vicinity of our former home in the galaxy and capture an image just as beautiful.

2012 Transit of Venus

2004 Venus Transit

2004 transit of Venus through a small telescope. Click image for source.

Next Tuesday, June 5th (June 5th in North America / June 6 eastern continents), you’ll have the opportunity to observe something that you’re extremely unlikely to ever see again. Over the course of a few hours, Venus will cross in front of the Sun from the vantage point of Earth. Venus will appear as a small black dot against the bright blazing disc of the Sun. Just like the annular eclipse from a couple of weeks ago, it is NOT SAFE to view this event directly. Here are a few ways to view it:

Disposable solar shade glasses – This is the cheapest and simplest method. These are the same glasses you would use to view a solar eclipse. They’re generally made of cardboard and have extremely dark film for lenses. When looking through them, you cannot see anything except for something as bright as the Sun. If you can see the surrounding landscape through them, they are NOT dark enough and you are at great risk of damaging your eyes.

Pinhole projection – If you’ve got clear skies and an overhead Sun, you can project the image of the Sun (and transit) using a simple pinhole projector. This can be as simple as a piece of paper with a hole poked in it, to a more elaborate and larger projector. Feel free to be creative, as long as you do it safely. Here are some sources for pinhole project ideas: Cosmos Magazine / TransitOfVenus.org / Exploratorium

Binocular/Telescope projection – You can also project a magnified view of the transit by using a pair of binoculars or a small telescope. Here, you want to point the objective lens (the big lens away from the eyepiece) at the Sun, let the light go through the binoculars/telescope and project that image onto a shaded piece of paper. Experiment with different distances until you get everything in focus. Note, that doing this method for a significant amount of time can damage the optics in your binoculars or telescope.

Webcast – If the clouds have you down or the transit occurs during your night time where you live, you can still watch the event unfold from what will certainly be a number of online webcasts. My friends at Cosmoquest will be hosting a Google+ Hangout with various feeds of the transit, and Slooh will make an event out of it as well.

So now that you know how to look, you need to know when and where.

Map showing where the 2012 Venus transit will be visible from.[Map showing where the 2012 Venus transit will be visible from. Source: NASA / Click for larger view.]

Being an amateur astronomer in Alaska (especially along the coast) is the true definition of optimism. There are a lot of clouds year-round, never-ending sunlight during the Summer, and frigidly cold winters that make skygazing a test of tolerance and wills. That said, on those few nights where the clouds have retreated, it’s dark, and above zero… those nights are a-maz-ing. Coincidentally, Alaska is a prime viewing location for the 2012 transit of Venus — in fact, the entire event will be viewable from up here. Ironically, I’ll be out of the state during the transit and will only be able to catch it during a North Dakotan sunset (which sounds pretty, anyhow).

For the most accurate information for your location, there are a handful of resources. There are free iPhone and Android apps for your smartphone. Additionally, if you can find your location on a map this webpage is a fantastic guide. An example of how it varies from place to place:

My home in Kenai, Alaska (June 5th):
Venus crosses into the limb of the Sun at 2:06pm local time. Approximately 20 minutes later, Venus is fully within the disc of the Sun. It will slowly make its way across the face of the Sun over the next 6 hours, reaching the opposite limb at around 8:30pm local time. At 8:48, the show is over with the Sun still high in the sky.

Where I’ll be in North Dakota (June 5th):
The transit will begin at 5:04pm local time. By 8:27pm local time, Venus will be at the center-point of its transit. Around an hour later, the Sun will set, taking the transiting Venus with it.

The bottom line is, due to the duration of the event you should be able to get at least a glimpse of it from anywhere in North America, to a varying degree as shown above. And you’ll definitely want to make every opportunity to see it, because it will quite likely be the last time you have the chance — unless, of course, you plan on being alive for another 105 years (and still have the eyesight to see it!). That’s right, this will not occur again until 2117 — so this is your chance.

Good luck and happy observing!


Upcoming Solar Eclipse

Readers located in the Western United States and East Asia should mark their calendars for this Sunday’s (May 20, 2012) solar eclipse. To some degree, the eclipse should be observable from Texas to Thailand, with certain locales observing an annular eclipse, while others will still get the treat of a partial eclipse.

Track of the May 20, 2012 solar eclipse overlaid on a map.

(Image courtesy of Google and NASA’s Eclipse Web Site)

 Note, you do not need to be on that path in the picture above to see the eclipse. If you’re within that path, you will see an annular eclipse. If you’re North of South of that path, you’ll see a partial eclipse. An annular eclipse is a solar eclipse in which the distance between the Earth and the Moon is great enough that it appears too small to completely block out the Sun. It will look like this:

Eclipse Anular

Photo of a 2005 annular eclipse, as seen from Spain. Photographed by Abel Pardo López and used under a Creative Commons license. Click image for source.

Unlike the case with a total eclipse, where the Moon is closer to Earth and covers the entire Sun (less its corona), do not look directly at an annular eclipse with your naked eyes!

For those of us that will be outside the path of the annular eclipse, many will still be able to see a partial eclipse. Without going too deep into the geometry of an eclipse, be aware that there are basically three types of shadows produced during the event: the umbra, antumbra,  and penumbra. The umbra is the darkest part of the Moon’s shadow, and when it falls upon the Earth it results in a total eclipse. From within the umbra, the Sun is completely blocked out by the Moon. From the vantage point of an observer within the penumbra, the Sun is only partially blocked by the Moon, resulting in a partial eclipse.  Within the antumbra, an observer will see the Moon pass completely between them and the Sun, however its apparent size compared to the Sun will be small enough that it will not completely block out the Sun; an annular eclipse.

The following diagram is a visual demonstration of what I’ve just described:

Diagram showing the different types of solar eclipses.

(Image Credit: University of Tennessee Department of Physics and Astronomy)

“Okay, but I just want to know if I can see it!”

Okay, so you want to know if you’ll be able to see the eclipse, and if so, when should you look? The best and simplest way to find out is to go to NASA’s Eclipse Web Site for this event. From there, you can click on your location on the map and a little window will pop up with details, like so:

Details of the May 20, 2012 solar eclipse for Kenai, Alaska

If you live near Kenai, Alaska (like me) there are your details. Take note that the times are in Coordinated Universal Time (UTC), so you’ll want to adjust accordingly based on your time zone. In my local case the eclipse will begin at around 3:15pm local time (UTC – 9 [AK Time] + 1 [Daylight Savings Time]) and continue for nearly three hours, as the Moon slowly moves across the face of the Sun. For other locations, you’ll find this tool very easy to use.

“What good is knowing when and where an annular eclipse is if I’m not allowed to look at it?!”

I’m glad you asked! By all means, do NOT look at this eclipse with your naked eyes. You will damage them. The visual part of the electromagnetic spectrum is far too beautiful to go damaging your instruments to see it (your eyes!). Fortunately, there are a few simple tools you can use to view it.

The most convenient method is to use cheap cardboard solar-shield glasses made specifically for this purpose. You can buy them online and elsewhere for less than a dollar. (Buy many and share! They’ll also be great for the Venus transit next month, but more on that later!) They look like this:

Solar shield glasses

If you choose to purchase some (there may not be enough time to receive them before Sunday, but there will be plenty of future opportunities to use them as well), I recommend purchasing through a company associated with Astronomers Without Borders, where proceeds will go to benefit others interested in astronomy. Make sure any you use are clearly labeled that they’re safe to view the Sun through.  An alternative to these glasses is to use Number 14 Welders’ Glass, available at welding supply shops.

You can also use a pair of binoculars or a telescope as follows, but make sure that nobody (small children, non-bright adults, pets) looks at the Sun through the eyepieces; it could very well be the last thing they see. To use binoculars or a telescope, you want to project the image onto a piece of shaded white paper. Just align the Sun with the objective lens (not the eyepiece lens) and let the light pass through and onto the piece of paper. An image of the Sun will appear on the paper and, while bright, will be safe to look at.

And finally, if you do not have solar shield glasses, Number 14 Welders’ Glass, binoculars, or a telescope (again, projected onto paper!), you still have another option. You can use a colander, a piece of aluminium foil with a hole punched in it, or even with the aid of a leafy tree. Obviously, if you took a colander outside on a sunny day, let the sunlight shine through it, and reflect onto the ground, you would see the circular dots of light where it was allowed to pass through the holes in the colander, and shade where the solid part of the colander blocked it. If you happen to do this when the Sun doesn’t appear as a solid circular light source (an eclipse), or if something is passing in front of it (a transit), the light in those dots on the ground will show it as well.

Check it out!:

Partial eclipse viewed with the aid of a colander.

(Image Source)

This same effect will work if you poke some holes in aluminium foil, a pizza box, or whatever you might have available. Luckily, you’ll have a bit of time during the eclipse to experiment and see what works best.

Even trees want you to see the eclipse:

The Eclipse Tree, Basildon.

(Image Credit: Picture Esk on Flickr)

So that about covers it. If you have any questions, if I’ve missed anything, or if you believe there is a mistake in my explanations, please leave a comment. I hope you’ll take the opportunity to enjoy this celestial treat and I hope you find people to share the experience with you as well.

Happy observing!


Celebrate Perihelion

Today, the Earth will be at a point in its orbit around the Sun called perihelion; the point in its orbit about which it is closest to the Sun. Until early July, we’ll be getting further and further away from the Sun, after which point we start getting closer again.

The overall change in distance is quite small, comparatively. Today, we’re approximately 3.1 million miles (just shy of 5 million kilometers) closer to the Sun than we will be in July, at aphelion. When you compare that to an average distance of around 93 miles, you’ll realize why the change in distance is virtually unnoticed by us Earthlings (unless we’re scientists specifically studying the Sun).

Perihelion