29 October 2016

In the olden days the first killing frost in Pittsburgh usually occurred by Halloween and we had to wear winter coats over our costumes while trick-or-treating.  … I always hated to cover my costume.

This year has been very warm, even hot.  Only ten days ago it felt like August and today the temperature is 5-10 degrees above normal.  When will we experience the first killing frost?

Some of you already have.  If you live east or north of Pittsburgh the growing season is shorter (bluer), as shown on the USDA Plant Hardiness Map below.  Bradford, Pennsylvania is three growing zones colder than Pittsburgh and the Monongahela and Ohio Valleys.  I’ll bet they’ve had a killing frost.

Here in Pittsburgh the weather forecast says we won’t dip near freezing for the next several days.  Today it’ll be 10 degrees above normal.

When do you think we’ll have our first killing frost?

(photo of frosty leaves by Dianne Machesney, excerpt from USDA Plant Hardiness map at USDA.gov)

The days are getting longer.  Really.

Though daylight is decreasing as we head into winter, the length of an Earth day is increasing overall. That’s because Earth’s rotation is slowing down due to tidal forces between Earth and moon, post-glacial rebound, and sea level rise.

The effect is too tiny to see.  It takes 100 years for the day to gain 1.4 milliseconds.  To put that in perspective, a day was 23 hours long for the dinosaurs and is close to 24 hours now.

The only way we can measure Earth’s rotation is by using an array of instruments stationed around the globe (VLBI) that precisely record their first sighting of certain quasars. We then crunch the data to arrive at the length of a day and add a second to our atomic clocks when necessary.

Want to learn how we measure a day?  See the video about quasars at this NASA link.

(*) Quasars emit radio waves so they aren’t actually seen, they’re heard.

(animation of earth’s rotation from Wikimedia Commons. Click on the image to see the original)

Something really big exploded 5,000 to 10,000 years ago and this is what’s left.

The Cyngus Loop or Veil Nebula is the dust and gas left over when a supernova exploded in the area of the Cygnus constellation.  The explosion was so bright that people could see it naked eye.

But they didn’t write about it.  The first written language was invented by the Sumerians 5,200 years ago, probably too late for anyone to mention a temporary bright spot in the sky.

Nowadays the remnants are too dim to see without a telescope and some sections such as the Witch’s Broom have been named separately.

The entire Veil, above, isn’t visible except in the ultraviolet range.  If we could see it we’d be impressed.  It’s six times the size of the full moon.

p.s. Many birds can see light in the ultraviolet range, so this is probably what the Veil looks like to them.

(ultraviolet image of the Cygnus Loop Nebula by NASA/JPL-CalTech via Wikimedia Commons. Click on the image to see the original)

1 October 2016

If it’s clear tonight in Pittsburgh you’ll be able to see the International Space Station (ISS) traverse the sky for six minutes.

At 7:41pm the ISS will appear in the southwest and pass directly overhead on its way northeast at 17,150 miles per hour.  At five miles per second it doesn’t take long to disappear.  Read more here in the Post-Gazette on what and where to look for it.

You don’t have to be in southwestern Pennsylvania to see it.  NASA’s Spot The Station website predicts ISS’s appearances around the world.

Armed with this information you can impress your friends.  Casually looking at the night sky you can say, “Look over there.  In half a minute the International Space Station will appear on the horizon and pass directly overhead.”

A fast-moving shiny thing in the sky.

(photo of the International Space Station as seen from the Space Shuttle Atlantis, 19 July 2011.  Click on the image to see the original)

p.s. Unless they’ve already fixed it, I believe there’s a typo in the Post-Gazette’s 2nd paragraph which says “Monday” but probably means Saturday.

THE BLACK MOON MADE ME FORGET SOMETHING I ALREADY KNEW.  BIG CORRECTION AT 10:45AM!  We never see the dark side of the moon except from outer space. Thank you, Tom Hoffman, for reminding me. (Shaking fist at Black Moon!)

30 September 2016:  Today the media is peppered with the ominous words “Black Moon.”  Here’s what that’s all about.

By the time you read this the moon has already risen in Pittsburgh at 6:46am. It came up half an hour before sunrise, will reach its zenith at 1:00pm, and will set at 7:07pm four minutes after sunset.  It’s in lock step with the sun.

But we won’t see it.

It’s a new moon traveling so close to the sun that the sun’s glare hides it.  And it’s not illuminated.  It is back lit by the sun.

This is the second new moon this month, the so-called “Black Moon.” Like its bright twin, the Blue Moon, two of these in a month are a relatively rare occurrence.  The last Black one was 32 months ago.

What about the dark side of the moon.  Is it always black?

No. Today’s Black Moon is sunlit on the other side.  But if we could see it, it would look unfamiliar.

During the new moon last July, the dark side of the moon was facing the DSCOVER satellite when NASA’s EPIC camera recorded time lapse photos.  Watch as the dark side flies by the southern hemisphere.  Doesn’t it look odd!

For starters, it’s darker than we expect.  Even when fully illuminated the moon is darker than Earth from outer space because it reflects less light.  Our planet is bright blue and white because it has lots of water.  The moon is dry and dark.  It matches the color of Australia.

Here’s the dark side in a still photo from August 2015.

Look closely and you’ll see that it’s missing the craters we always see.

Of course we shouldn’t expect the dark side to match the bright side.  But the fact that it looks so different makes this Black Moon unsettling. 

(New moon symbol from Wikimedia Commons. Moon and earth animation from NASA.  Click on the images to see the originals.)

It’s not the heat, it’s the humidity.  But its not the relative humidity.(*)

As a raw number, relative humidity doesn’t tell you anything.  The video above shows how the same amount of water produces different relative humidities depending on air temperature.

For example, early yesterday morning in my backyard it was 80 degrees with relative humidity 79%.  Last Tuesday it was 66 degrees with relative humidity 83%.

So didn’t yesterday’s 79% humidity feel better than 83% last Tuesday?  No!  Yesterday’s 80 degrees held a lot more water.

Dewpoint (the temperature at which the air is so saturated that it rains or produces dew) is the helpful number that tells us that.  If you know the temperature and relative humidity you can calculate the dewpoint here.

The National Weather Service in Chicago made a chart to describe how we feel at various dewpoints.  I’ve marked it in red to show my own heat-averse opinion.  (Click on the screenshot to see their dewpoint video that includes this chart.)

So here’s what was really going on this week and why it felt so hot yesterday even though the temperature never reached 90 degrees.  Notice that the relative humidity was at its lowest yesterday afternoon.

Find out the dewpoint before you go outdoors and you’ll know whether you want to brave it!

(*) p.s. See the comments!

(video from Richard Clements on YouTube. screenshot from NWS Chicago video. Click on the screenshot to see the video)

June 20, 2016:

Today is astronomical.  It’s been 68 years since we’ve seen one like it.

The last time the summer solstice occurred on the same day as the full moon was in 1948.

By the time you read this the moon will have done its job, having reached maximum fullness at 7:05am in Pittsburgh.  It had already set by then (6:13am) so we didn’t see it.

The summer solstice is yet to come — 11.5 hours after the moon’s event — at 6:34pm.

The moment when the sun stands still is such a big deal that they’re celebrating it with a four-day solstice festival at Stonehenge, pictured above.  But they won’t be able to see the sun during its special moment.  It’ll be almost midnight at Stonehenge, 11:34 pm.

Read more about this astronomical event at the Old Farmer’s Almanac.

(photo from Wikimedia Commons. Click on the image to see the original)