Repost: Sky Watcher’s Astroblog 3

Happy September.  September means two good things (at least): the beginning of nights with no mosquitoes and it’s getting dark earlier.  I know that the shorter days are not generally a popular thing to call “good”, but it means that there are more awake-hours to spend out under the night sky.  Two more things of interest are the Harvest Moon and the Autumnal Equinox.

The Harvest Moon, besides just being a great excuse to dance with someone special out under the stars, is the full moon that occurs closest to the Autumnal Equinox (more on just what the equinoxes and solstices are, we’ll talk about later).   This year, with the Equinox occurring on the 23rd, the dates of the full moon that fall on either side of it are September 12th (11 days before the Equinox) and October 11th (18 days after the Equinox).  So, the Harvest Moon is on September 12th (enjoy your dance!).  What makes the Harvest Moon special?  The short answer is that for a few days in a row, that big, bright, nearly full moon is in the sky and ready to take over lighting fields by the time the Sun has completely done its job for the day, giving farmers more time to bring in their crops.  The (slightly) longer answer is that the path the Moon (and the Sun, the planets and most asteroids) follows across the sky (called the Ecliptic) makes a shallow angle with the Eastern horizon at this time of year.  That may be a bit hard to picture so, here’s a picture (the circles represent the Moon on successive days and the tilted lines are the Ecliptic):

As the Moon orbits the Earth, it moves along the Ecliptic from West to East (right to left as we see it in the sky).  Since it takes the Moon about 30 days to move from Full Moon to Full Moon, it moves 1/30 of the way along in its orbit each day.  This means that the Moon rises later and later each day.  Our days are 24 hours long.  On the equator the Ecliptic is exactly vertical to the eastern horizon at the equinoxes and, if the Moon was near full then, it would rise 24/30 of an hour later (48 minutes) than it did the day before.  But, as the Earth moves in its orbit around the Sun, the angle between the Ecliptic and the eastern horizon at moonrise changes throughout the year.  When the Ecliptic is at its steepest angle at moonrise (at the Vernal Equinox) the daily rising of the Moon differs the most from day to day (about 84 minutes).  When the Ecliptic is at its shallowest angle to the horizon at moonrise (at the Autumnal Equinox), the time between moonrise each day changes the least (about 24 minutes).

Sky Watcher’s Astroblog 15

Hello and Happy Summer!

Alas, I must begin with the news that I was clouded out of seeing the transit of Venus on the 5th.  That was very disappointing to me (and many others).  I was home that day (on vacation) and throughout the morning, I monitored the National Oceanic and Atmospheric Administration website (noaa.gov) watching for a break in the cloud cover.  Hoping until the end, I packed a couple of telescopes in the car and headed out for the afternoon ready to setup anywhere I was if the Sun peeked through the clouds.  It didn’t happen.  Of course, as is often the way with these things, later that night the stars were out!  A friend was visiting so at least we took a quick look at Saturn.  Well, there’s just under 105 years left to go until the next one.

As I mentioned in my last blog entry, the Summer Solstice will occur on the 20th at 7:09 p.m. (whether it’s cloudy or not!).  At that moment, the Sun reaches its most northerly position over the Earth for the year

When the Sun finally does go down after these long days, be sure to go outside and take a look at Saturn, Mars and for a few more days, Mercury.  Start with Mercury, since it sets before the other two.  Mercury is the closest planet to the Sun and if you can remember some of the diagrams we used a couple of months ago, you’ll realize that because Mercury is closer to the Sun than the Earth is, it never appears very far away from it in our sky.  That means we never get to see it in a really dark sky, but only just after sunset or just before sunrise.  If you have an open view to the West, look low on the horizon just after the Sun goes down.  How low?  Reach your arm out to the spot where the Sun disappeared, make a fist with your thumb at the top and look in the sky near the top of your fist and maybe a smidge to the left.  The first point of light you see in that area as the sky darkens will be Mercury.  Mercury will be at its greatest separation from the Sun (and so easiest to see) on the 30th.  This is called its greatest Western Elongation.  After that, it will start to set earlier and earlier each night and then, like Venus did, will get too close to the Sun to see for a few days and then reappear in our morning sky.

After Mercury sets and it continues to get darker, look towards the South and Southwest to find Saturn and Mars, respectively.  Saturn is the bright yellowish-white object just above a bright star and Mars is off to the right about three-fist-lengths away.  Over the course of the next couple of weeks, we’ll see Mars moving closer and closer to Saturn.  On the 27th, they will be joined by a crescent Moon for a very pretty picture!

That’s all for now.  Enjoy the summer evenings and LOOK UP!

Barry
bdc13@comcast.net

Sky Watcher’s Astroblog 14

Hi, everyone.  I hope you have made your plans to view the transit of Venus on the 5th, if possible.  I’ll share any pictures that I take of the event in a future blog.  Speaking of pictures, you might have heard that there was an eclipse of the Sun a week or so ago.  I didn’t mention it because it wasn’t visible from where we live.  You had to be in theWestern U.S. (at least) to be able to see any of it.  However, my good friend Pat was in Nevada at the time and went prepared with a camera and the proper filter.  He has kindly given me permission to share his pictures with you!

            

 Notice a couple of things in the mid-eclipse picture.  First, the shadow of the Moon is not centered on the Sun as in some of the pictures you might have seen.  That’s because Pat’s location wasn’t directly under the Moon’s shadow (black circle) – it was off to one side, a few miles away.  Second, if the Moon WERE centered, would it completely cover the Sun’s disk?  No.  On average, the Sun and the Moon appear to us to be about the same size.   But for this eclipse, the Moon appeared a bit smaller than the apparent size of the Sun.  Let me explain.

Remember last time when I talked about May’s Full Moon appearing especially big?  It was because the Moon’s orbit is elliptical and the Moon was a bit closer then.  Now, with the Moon half-way around in its orbit from where it was two weeks ago, the Moon is about as far away from the Earth as it can get.  When things are farther away from us, they appear smaller.  In this case, the effect is that the Moon appears smaller to us than the Sun and doesn’t completely block it from view.  If the Moon had lined up just right with the Sun from where Pat was, he would have seen a complete ring of sunlight around the Moon.  Thanks again for the pictures, Pat!

The next Full Moon occurs on June 4th.  It’s known as the Rose, Flower or Strawberry Moon.  Now, also in my last blog, I said that Venus’ orbit around the Sun was tilted with respect to the Earth, and only when the orbits intersected AND Venus was between the Earth and the Sun do we get to see a transit.  Likewise, the Moon’s orbit is tilted with respect to Earth’s equator and only when the two intersect do we get to see eclipses.  This means that solar eclipses (when the Moon is between the Sun and the Earth and we move through the Moon’s shadow) and lunar eclipses (when the Earth is between the Sun and the Moon and the Moon moves through the Earth’s shadow) occur in pairs.  So, at the New Moon on May 20th there was a solar eclipse and now that the Moon has moved half-way around in its orbit there will be a lunar eclipse at this next Full Moon.  Unfortunately, once again we won’t be able to see it here in the Northeast. (The Moon won’t be up—above our horizon—when the eclipse occurs.)

Finally for June, the Summer Solstice occurs on the 20th at 7:09 p.m.  Looking back, in my 3rd blog when I talked about the Autumnal Equinox I said, “…more on just what the equinoxes and solstices are, we’ll talk about later…”.  Well, later is ALMOST here.  This entry has gone on long enough.  I’ll be back in a week or two to cover that one.

Until then, enjoy the summer evenings and LOOK UP!
Barry

Sky Watcher’s Astroblog 13

Hello again.  It’s finally starting to warm up and stay warm, and hopefully, that will induce us all to go outside at night more frequently and look at our sky.  Before I point out things to look for, I want to tell you about the transit of Venus that will occur on June 5th.  This will most likely be your last opportunity to see one.  A transit occurs when an object crosses the face of the Sun as seen from Earth.  Since Venus is closer to the Sun than the Earth, it passes between the Earth and the Sun on a regular basis.  In fact, this happens every 584¼ days (1.6 years).  So, you might expect that every 584¼ days, we would be able to see Venus transit the Sun.  But we don’t.  The orbits of the Earth and of Venus around the Sun don’t lie in the same plane – they are tilted with respect to each other by 3.4 degrees.  In our skies the Sun only takes up ½ of a degree so most times when Venus passes between us and the Sun, it does so either above or below the Sun.  Only rarely when it passes between us and the Sun does Venus line up just right so that it passes in that ½-degree band.   How rarely?  The number of years between transits varies in a cycle.  The cycle goes like this:

8 yrs / 105 yrs / 8 yrs / 113 yrs / 8 yrs / 105 yrs / 8 yrs / 113 yrs / …

Get the picture?  The last transit was in 2004, eight years ago.  The next transit won’t be for another 105 years.  I’m  guessing I won’t see that one – even if the skies are clear that day!

To read the rest of Sky Watcher’s Astroblog 13, click HERE.

Sky Watcher’s Astroblog 12

Hi.  Just a short note to tell you about exciting astronomical happenings around my house.  Last night (the 24th) with the help of my wife, Carolyn, I ran an astronomy program at the Soule Homestead in Middleboro.   The clouds kept us from going outside to look through my telescope, but we had a lot of fun with the indoor portion of the event.  Using a map of the stars that would have been visible outside if it had been clear, about two dozen adults and children devised their own constellations and shared them with the group.  The result was the Soule Homestead Spring Star Map!  The point was it’s great to know the “official” constellations, and you need to if you’re going to talk about the stars to others, but you can still enjoy the night sky and make it your own by picking out stars or groups of stars and recognizing them as something that makes sense to you.  Kings, queens, charioteers and dragons aren’t people and things that most of us have any connection with.  The constellations we plotted were the Bear, the Euglena, the Question Mark, the Trigger Fish and the Rabbit (or Mouse) – all objects that at least one person in the group “recognized” in the stars.  Then we explored the motions of Venus and Jupiter as I’ve been describing in the last couple of blogs using software animations to help us watch the planets move.  A third topic that came up, the signs of the Zodiac, is something I’ll discuss with you all in a future writing.  We enjoyed our visit to the Soule Homestead and hope to return soon on a clear night to do some viewing.

The second big event around my house has been the publication of Carolyn’s book, A Black Hole Is NOT a Hole.  The book explains black holes in a way that they are seen as really interesting places in space as opposed to big, scary things that lurk “out there” waiting to suck in everything.  It’s written for 9 to 14 year-olds and all the adults that I’ve talked to think it’s just right for them, too.  Carolyn’s official book launch took place last Tuesday at the Wellesley Booksellers with readings from the book and audience participation to bring some aspects of black hole properties down-to-Earth.  That event was followed by a party at a friend’s apartment nearby.  In the future, Carolyn will be doing other readings around the local area.  The day before the Wellesley gig, Carolyn was interviewed live on the Vic MaCarty Show on WMKT radio from Michigan.  The book has received starred reviews from several national reviewers.  It’s also on the National Science Teachers’ Association’s Recommended list.  Sorry if this seem like a shameless plug, but it HAS been a very exciting (and busy) week around here!

When I write again about what’s up at night, I’ll give you more details on the June 5th transit of Venus, as promised.

Until then, Clear Skies!
Barry — bdc13@comcast.net

Sky Watcher’s Astroblog 11

Hello.  I hope you have been checking out the sky on clear nights and watching the Jupiter-Venus-Moon action.  I also hope my last blog clicked for you.  When I ended my last blog, I said that on the night of the next Full Moon (March 7th), the Moon would be nowhere near Jupiter and Venus and showed a diagram with Jupiter and Venus a bit closer together then they were at the end of February when the Moon was near them. I promised to explain why and here’s the explanation.

To read the entire astroblog #11, including illustrations to gain a better understanding of current activity in our solar system, click here –> Astroblog Installment #11

Sky Watcher’s Astroblog 10

Hi. I hope everyone’s recoved from your Imbolc celebrations.  Now, back to sky-watching. Last blog, I ended with telling you that I’ve been watching the positions of the Moon, Venus and Jupiter.  I hope some of you have been, too.  As I write this (February 27th), the Moon, Venus and Jupiter are all in the same area of the sky and are shifting their positions relative to each other.

To read the entire astroblog #10, including illustrations to gain a better understanding of current activity in our solar system, click here –> astroblog installment10.