The past importance of transits of Venus

A French cartoon showing a view of the 1761 transit of Venus.  Note the devil in the background, which I assume is meant to remind the reader that viewing the transit through a telescope without proper eye protection is a deadly sin (at least for your retina)
Image Credit: NASA Goddard Space Flight Center Sun-Earth Day

Next week's transit of the planet Venus across the sun gives us the chance to learn a little history of transits and their scientific importance.  In a couple days, I'll discuss the current scientific interest of transits.

The importance of Venus transits starts with the famed astronomer Johannes Kepler, who, in the early 1600s, was the first person to figure out the shape and properties of planetary orbits.  His three laws of planetary motion allowed Kepler to figure out the relative size of the solar system.  If we call the average distance between the Earth and the Sun as 1 Astronomical Unit, then Kepler knew the distances to other planets in terms of this unit.  For example, he knew that Venus was about 0.7 Astronomical Units.    The problem was, Kepler didn't know what an astronomical unit was in terms of familiar units like miles or kilometers. 

Live in northeast Texas? Come see the transit of Venus with us!

2004 transit of Venus, photograph (c) Scott Thompson

If you live in northeast Texas, you are invited to come watch the Transit of Venus at the Texas A&M University - Commerce Observatory in Commerce, TX.  We will get underway at 5 p.m. on Tuesday, June 5 at the observatory, which is about 5 miles north of Interstate 30 exit 101 or 5 miles south of Commerce.  Details and driving directions are here.  The transit will last until sunset (awesome photo opportunity!).  We will have all the safety equipment you will need to get great up-close (and safe) views of the transit.

After sunset, the party doesn't end!  As it gets dark, we will turn the telescopes on the wonders of the spring and summer skies.

The 16" telescope dome

In case of poor weather, the observing will be cancelled.  Check the observatory webpage for updates or watch my Twitter feed.

Coming June 5/6: The Last Transit of Venus You'll See

(c) 2004 Fred Espenak

It is rare that you can see something and know you will never see it again.  A week from today (June 5 or 6, depending on where you live: June 5 in the western hemisphere including the U.S., June 6 in the eastern hemisphere) you have that chance when the planet Venus crosses in front of the sun as seen from the Earth.  The next time this happens will be in December 2117.  That's not a typo - you have to wait 105.5 years for another crossing.

Sunday's "Ring of Fire" Solar Eclipse

This Sunday, May 20 (in the U.S.; Monday morning on the 21st for Asia) there will be a spectacular eclipse of the sun.  Residents of the western U.S. get a great show; those on the Eastern seaboard get to see nothing.  Details on how to see the eclipse can be found here from Sky & Telescope and on many other websites.  Rather than reproduce others' details on how/where/when to look, I thought I'd put a personal spin on the story and mention a few things I haven't seen on many other websites.

Solar eclipses are caused when the moon comes between the Sun and the Earth; basically we see the moon's shadow.  The moon's orbit is tilted with respect to the Earth, so most months the moon passes well north or south of the sun in the sky, but every 6 months it has a chance of passing over part or all of the sun.

The moon and the sun are almost exactly the same size, as seen from the Earth.  But the moon's orbit is not a circle, it is elliptical (oval), so sometimes it is a little closer to the Earth and sometimes a little further away.  Remember a few weeks ago when the "Supermoon" was big news?  I was in an ice cream shop when one of the other patrons saw the full moon rising and shouted "It's the supermoon!  Look how big it is!  Let's all go look!  Supermoon!".  You can see some pictures of the Supermoon here.   The reason for the "supermoon" was that the full moon was almost exactly coincident with the moon's closest approach to Earth, so it was fully lit at the same time it appeared largest in the sky.