Jan 022013

I know it was the coldest morning of the winter so far today but the earth will reach perihelion, a point in its orbit where it is closest to the sun.

This may strike you as odd, wouldn’t the earth always be the same distance from the sun?  Well, it would if the earth traveled in a circular orbit.  Instead, the earth travels around the sun in an ellipse.

So this means there is a farthest point too, called aphelion, which occurs in early July.  The difference is about 5 million kilometers with perihelion around 147 million km and aphelion around 152 million km.

By Anthony Ayiomamitis

This amounts to about a 3.4% change.  You wouldn’t normally notice this but it can be measured as in the included image shows.

This has nothing to due with the seasons which are caused by the earth’s 23.5 degree tilt. But you would think there would be some noticeable effect, after all it’s 5 million kilometers!  It’s turns out there is an effect but it is small and just about masked out by the tilt and by changeable weather patterns.

Also, there is no relationship between perihelion and the winter solstice, which are presently separated by about two weeks. In fact, the two are drifting apart!  In the year 1246 AD, they were coincident. Around 6000 AD, perihelion occur at the same time as the Vernal Equinox in March.

Crazy isn’t it?  Milankovitch thought so too!

Sep 222012

Sun Path at the Seasons

There is often much to do about the official start of seasons in general and Autumn of course today.  So what actually happens weather-wise on the day of a season change?  Short answer: not much!

It’s not even a day really. The astronomical start to each of the four seasons is actually a point in space and time in the Earth’s orbit around the sun.  It happens at a precise time.  In the case of today, the Autumnal Equinox starts at 9:49 am CDT.

The most significant thing about today is the length of day and the position of the sun. The length of the day is twelve hours everywhere on the planet.  It would be this way everyday if our planet were not tilted on it’s axis at 23.5°. But the tilt provides a more complicated geometry to the path of the sun across the sky which then affects the length of the day.

The enclosed diagram shows the arc length of the sun at the four seasonal points.  Summer has a long arc length and the highest sun angle therefore we have the best quality and quantity of sunlight at this time. Winter has the shortest sun arc length across the sky and the lowest sun angle therefore more limited energy.

Notice at the two equinoxes, the sun rises and sets in due east and west.  This has consequences to drivers on east-west roads around sunrise and sunset. At these times, the sun would be right in your eyes on such streets making visibility of traffic, traffic lights and pedestrians almost impossible. Be careful this evening and for the next few days!

This day really has no practical affect on the weather. Again it just a point in space along what has been a gradual shift in sun angle and path which started back on the Summer Solstice.  Too subtle to have a direct effect on daily weather!

Another good write-up on the seasons can be found here.




Aug 102012

Perseid Meteor Shower Viewing

The viewing conditions for a summer sky event, the Perseid Meteor Shower, should be great on Saturday-Monday when the peak meteor count is expected.

The much-needed break from the heat is being supplied this weekend by a center of high pressure coming in from the north.  Most highs do a great job of supplying not only cloud-free conditions but also bringing in a transfusion of cleaner, less humid air leading to great sky-watching!  Also, a crescent Moon means very little meteor-masking light.

The Perseids are the premier meteor shower of summer occurring in August. They are caused by earth’s orbit traveling through the debris path of comet Swift-Tuttle.

According to this NASA write-up, the early morning of the 13th should provide the best show with a waning Moon and two planets coming close to each other!



Jun 202012

The Seasons

Summer begins, astronomically, at around 6:10 pm CDT today, Wednesday, June 20th.  This time marks a point in the earth’s orbit around the sun.

It is significant because the earth is tilted slightly as in spins, 23.5 degrees to be precise. Because it’s always tilted in the same direction relative to the plane of the earth’s orbit,  the Northern Hemisphere (everything north of the equator) will be tilted at the sun  (summer solstice) on one extreme and away from the sun (winter solstice) on the opposite side of the orbit.

Tilt defines the seasons.  A sun which appears higher in the sky can direct more energy per square unit area than a sun with lower angle. Also, lower angles mean the sunlight has to pass through more atmosphere, leading to more energy loss.

This is the longest day of year and it also has the shortest shadows.

Weather-wise, not much happens!

I recorded a lecture this morning for my summer session students which you might find useful:

Jun 042012
NASA Image

On Tuesday, June 5th, the planet Venus will do something it rarely does…it will visibly pass between the sun and earth.  This means you can see it pass in front of the sun in what is called a transit.  Venus is the second planet and closer to the sun which is what makes this possible.

This will appear as something about the size of a sunspot only it is a transit so it will trace a path across the face of the sun.  In the Ozarks, this will happen close to sunset making viewing somewhat easier.

Obligatory comment: don’t stare directly at the sun!  You can use an approved filter or #14 welding glasses for proper protection of you eyes.  I enclosed a NASA video which does a good job of explaining viewing and other interesting facts about a Venus transit.

It will be difficult to see without magnification since it will only cover a snall fraction of the sun’s surface. Perhaps the best method is to watch it online. Here’s a link to do just that:

NASA Live Venus Transit

The transit starts at about 5:04 CDT and will go until the sun sets.

Here’s a nice write-up by FOX News.

May 152012

Eclipse Path

This Sunday, May 20th, some folks in the western U.S. will be treated to an annular eclipse of the sun.  For everyone in the Ozarks, this will amount to a partial solar eclipse starting before sunset on Sunday and ongoing as the sun sets around 8:19 pm

Before I write another word, please be advised that even an eclipsed or setting sun can do permanent damage to your eyes if you look without the aid of a filter or some other indirect viewing device! Standard sunglasses are useless.

See this site for a list of safe methods of viewing a solar eclipse.

What will you see?  Well, starting around 7:27 pm, the moon will begin to obscure the sun.  Right at sunset, it will cover around 2/3rds of the area of the sun.  This is worth seeing if you ask me!

Partial Solar Eclipse

It might look something like the included picture right around 7:50 pm or so.

An annular eclipse occurs when the moon is at apogee, the farthest it can be in orbit from earth.  Because it is smaller in the sky, it cannot cover the entire disk of the sun.  Those treated to this site will see the “ring of fire” of the sun around the moon.  In the United States, the total effect will be seen in a line roughly from northern California to western Texas.

The weather may be a factor.  Another front will be paying as visit to the area on Sunday.  Showers will be developing nearby and the clouds may obscure the view!  I’ll keep you posted.

Here’s a link to a nice NASA video of this and other astronomical events in May.

Here’ s the link to the Solar Eclipse Calculator




May 042012

How Super Will the Moon Be?

So what is all of this about a “Supermoon”?

Well, there is nothing new in the way the moon orbits the earth but there is lots of talk about supermoons lately so let me explain why you might notice something different about Saturday’s full moon.

Our official full moon is Saturday night at 10:36 pm CDT. It will rise right around 8 pm. Now, for full moons rising at this time, they can often be quite bright and large (the large apparent size of the moon on the horizon, called the “moon illusion”, is an interesting subject on of its own!).

Now, a so-called “supermoon” occurs when the moon’s orbit is closer to the earth. Yes, it does vary between perigee, closest to the earth and apogee, farthest from the earth. The moon orbits in a slight eclipse, not in a circle.

Doing the math on the distance from the earth to the moon yield about a 16% brighter moon. This might not be noticeable. However when the moon is first coming up these next few evenings, the combination of the “moon illusion” with a closer moon might just cause you to notice!

I’ve always been quite impressed with the moonrise visibility on the east side of Springfield along Highway 65. It looks like the weather will hold up, with very few clouds expected.


Jan 232012

From Space Weather

Late last night, the surface of the sun burst out a high-energy solar flare, classified as an M9.

Sunspots are the source for these energy surges.  While what drives the intensity and timing of the flares is not completely understood, the position of the sunspots can be tracked and in fact are reported daily on the Space Weather site and other sites as well such as the Space Weather Prediction Center.

This major event emanated from spot 1402.  Note that there are two factors which effect whether a solar flare will strike the earth.

First, how close was the culprit sunspot to the orbital plane?  If the sunspot in question is closer to the equator of the sun, it is more likely to affect the solar system. 1402 directed much of its energy “north” of the earth and somewhat out of the orbital plane.  But in this case, the strength is such that even a glancing blow should send a lot of energy our way!

Also, was the sunspot pointed at earth when it erupted? Our sun rotates but because it’s not solid, it does so at different rates depending on whether you are at the equator (about 25 days) or at the poles (closer to 30 days). So the sunspots move along the surface and if they are pointed away from earth when they erupt, they won’t affect our planet.

This latest solar flare, which is the strongest reported since May of 2005, is forecast to arrive around 8 am on the 24th, Tuesday morning.  The daylight timing will not work out well for aurora viewing but there might be plenty of energy left by nightfall.  BUT clouds might get in the way here in the Ozarks!

Jan 222012


Solar WInd 4th Line Down

The CME which was forecast to strike earth yesterday did finally make an impact today around 6:17 am but with less intensity then forecast. Space weather forecasters still believe viewing will be possible tonight once again.

 The solar wind did increase upon the arrive of the CME but not to particularly intense levels.  At this writing, it was “blowing” around 422 km/sec, stronger than first contact levels.  If this keeps up, it may again be possible for Northern Lights tonight.

As for our weather for viewing, a cold front will sweep in late this afternoon with some drier air and clearing behind it.

Jan 212012

Matt Rhodes, Pea Ridge, AR. 10/24/2011

[UPDATE] has reported that the CME due to strike the earth is overdue. It could still happen.  Some Northern Lights have been reported tonight at high latitudes.


Despite some clouds still hanging around this morning, clearing skies are expected later today and early this evening, making it possible to view the expected Aurora Borealis.

Forecasting the exact arrival time and intensity of these blasts of particles from the sun called Coronal Mass Ejections (CME’s) has gotten better but is still not precise.  Space Weather is still calling for arrival at earth around 4:30 local time with a several hour margin of error.  But since we’re only concerned about anything after sunset, our hope would be that the forecast is exact or perhaps early by several hours.

We had a decent CME back on October 24th with reports of the Northern Lights as far south as Arkansas and the Carolina’s.  They were observed in the Ozarks as well.  I did a report on Auroras at this time:

At this latitude, the Northern Lights are not likely to be noticed casually.  You’ll have to make an effort to optimize viewing.  This includes getting as far away from lights as possible and having an unobstructed view of the northern horizon, i.e. n0 trees or hills.

Also, many photographs showing an Aurora feature long time exposures which make for a more vivid image than what might be visible to the naked eye.  Something to keep in mind when viewing.

So, anytime after dark tonight.  We’ll see what shows up!