Starry Night 7, eclipses and ... Elmo

Like most two years olds, my daughter loves Elmo. Something about his high-pitched voice and ever-present smile I guess.

Parents innately learn to tune him out, probably to moderate the risk of going insane listening to that same shrill voice on reruns day after day.

One episode that sticks out in my mind, today more than ever perhaps, puts Elmo in a room with none other than David Beckham. The short ends after Elmo, who pesters Becks to teach him to dribble a soccer ball on his head, learns a new word... Persistence.

Persistence is an extremely important quality, and some might say a disappearing quality these days. It goes hand-in-hand with patience I think, something we can all agree is becoming scarce in this age of on-demand everything.

Persistence, as in the case of Elmo and Becks, often pays off.

Over the past few weeks, I've been involved in an exchange over the accuracy of Starry Night's eclipse predictions. One of our "power users" (who shall remain nameless for now) reported what he believed to be an issue with our calculations. They just didn't match up exactly with what NASA (and other government sites) were reporting.

The discrepancy was small; in fact, the timings were exact, to the sub-second but the sizes of the Earth's shadows (both the umbra and penumbra) were too small by a very small but noticeable fraction.

Normally no-one would even notice this, but in this case it changed our prediction for the April 4th, 2015 eclipse from a Total (as reported by NASA) to a Partial eclipse. That made all the difference to a few people.

I sit down at my 'puter. "I'll figure this out", I say to myself confidently:

  • Were we perhaps using an old or imprecise value for the Earth's diameter? Nope. That was in agreement with the USGS.
  • Were we taking the oblateness of the Earth into account? Yup. A precise calculation there too.
  • Was this somehow rounding or another error in calculation? Nope. Everything seems to check out.

After a few hours of this, I checked with my confrere, Bill (our SkySafari developer) to see if they had the same discord with NASA predictions. They did! And even better (worse, it turns out) both Starry Night, and SkySafari, two completely independently created applications, gave almost exactly the same result!

That sealed it for me. Other matters were more pressing. NASA could be wrong too (note that it's not insane to believe this, the pages we were referring to were intended for the public, maintained by a summer student and weren't necessarily vetted by anyone), or could have used slightly different values for the radius of the Earth. Were THEY accounting for the oblateness? Doubts and lack of information made it difficult.

After presenting my findings to Keiron, our head of support, I let the issue go.

Lots of other stuff to work on.

Again, the power user, and Keiron came back to me. Why were our predictions different? We claim arcsecond precision in our planet and moon positions. How could we make that claim if this discrepancy existed?

Again, I chatted with Bill. He said he had been talking with Keiron too. I Don't know what this annoying user thinks he knows that we don't. What could it be? "The Earth's atmosphere?" Says Bill. I dunno. Does NASA take it into account? It's not clear from their site. How can we compare if they don't say? The discrepancy is so small.

Lots of other stuff to work on.

Again, an email from Keiron. "Aren't you busy?" I think to myself. I open it, a curt reply, ready at my fingertips. This time, the message comes with a link.

"Do you think the Danjon rule explains the difference?" he asks:

http://www.eclipsewise.com/oh/oh-help/LEshadow.html

...

Holy s*&t!

...

Not just a clear explanation of what, how, and by how much the Earth's atmosphere contributes to the shadow, but authored by Fred Espenak himself, The Godfather of eclipse predictions! 

This page on the eclipse, with nicely outlined parameters showed something called "Shadow Enlargement":

http://www.eclipsewise.com/lunar/LEprime/2001-2100/LE2015Apr04Tprime.html

... with a link to a clear explanation of what the Danjon shadow rule is, and its value.

It turns out that the Earth's atmosphere contributes around 1/85 (just over 1%) to the diameter of the Earth's shadow... an effect that we were NOT taking into account in Starry Night or SkySafari... but are now! (SN version 7.1.2+, SkySafari ).

Starry Night has been around for almost 20 years in some form or another, and we have never taken this into account, until now.

We could all learn a little from this power user and Keiron (and Elmo for that matter). It pays to be persistent! 

Thanks for keeping at it guys. Starry Night is that much better for it.

After a few hours of this, I checked with my confrere, Bill (our SkySafari developer) to see if they had the same discord with NASA predictions. They did! And even better (worse, it turns out) both Starry Night, and SkySafari, two completely independently created applications, gave almost exactly the same result!

That sealed it for me. Other matters were more pressing. NASA could be wrong too (note that it's not insane to believe this, the pages we were referring to were intended for the public, maintained by a summer student and weren't necessarily vetted by anyone), or could have used slightly different values for the radius of the Earth. Were THEY accounting for the oblateness? Doubts and lack of information made it difficult.

After presenting my findings to Keiron, our head of support, I let the issue go.

Lots of other stuff to work on.

Again, the power user, and Keiron came back to me. Why were our predictions different? We claim arcsecond precision in our planet and moon positions. How could we make that claim if this discrepancy existed?

Again, I chatted with Bill. He said he had been talking with Keiron too. I Don't know what this annoying user thinks he knows that we don't. What could it be? "The Earth's atmosphere?" Says Bill. I dunno. Does NASA take it into account? It's not clear from their site. How can we compare if they don't say? The discrepancy is so small.

Lots of other stuff to work on.

Again, an email from Keiron. "Aren't you busy?" I think to myself. I open it, a curt reply, ready at my fingertips. This time, the message comes with a link.

"Do you think the Danjon rule explains the difference?" he asks:

http://www.eclipsewise.com/oh/oh-help/LEshadow.html

...

Holy s*&t!

...

Not just a clear explanation of what, how, and by how much the Earth's atmosphere contributes to the shadow, but authored by Fred Espenak himself, The Godfather of eclipse predictions! 

This page on the eclipse, with nicely outlined parameters showed something called "Shadow Enlargement":

http://www.eclipsewise.com/lunar/LEprime/2001-2100/LE2015Apr04Tprime.html

... with a link to a clear explanation of what the Danjon shadow rule is, and its value.

It turns out that the Earth's atmosphere contributes around 1/85 (just over 1%) to the diameter of the Earth's shadow... an effect that we were NOT taking into account in Starry Night or SkySafari... but are now! (SN version 7.1.2+, SkySafari ).

Starry Night has been around for almost 20 years in some form or another, and we have never taken this into account, until now.

We could all learn a little from this power user and Keiron (and Elmo for that matter). It pays to be persistent! 

Thanks for keeping at it guys. Starry Night is that much better for it.

Again, I chatted with Bill. He said he had been talking with Keiron too. I Don't know what this annoying user thinks he knows that we don't. What could it be? "The Earth's atmosphere?" Says Bill. I dunno. Does NASA take it into account? It's not clear from their site. How can we compare if they don't say? The discrepancy is so small.

Lots of other stuff to work on.

Again, an email from Keiron. "Aren't you busy?" I think to myself. I open it, a curt reply, ready at my fingertips. This time, the message comes with a link.

"Do you think the Danjon rule explains the difference?" he asks:

http://www.eclipsewise.com/oh/oh-help/LEshadow.html

...

Holy s*&t!

...

Not just a clear explanation of what, how, and by how much the Earth's atmosphere contributes to the shadow, but authored by Fred Espenak himself, The Godfather of eclipse predictions! 

This page on the eclipse, with nicely outlined parameters showed something called "Shadow Enlargement":

http://www.eclipsewise.com/lunar/LEprime/2001-2100/LE2015Apr04Tprime.html

... with a link to a clear explanation of what the Danjon shadow rule is, and its value.

It turns out that the Earth's atmosphere contributes around 1/85 (just over 1%) to the diameter of the Earth's shadow... an effect that we were NOT taking into account in Starry Night or SkySafari... but are now! (SN version 7.1.2+, SkySafari ).

Starry Night has been around for almost 20 years in some form or another, and we have never taken this into account, until now.

We could all learn a little from this power user and Keiron (and Elmo for that matter). It pays to be persistent! 

Thanks for keeping at it guys. Starry Night is that much better for it.

Partial solar eclipse on October 23rd

On the evening of October 23rd a partial solar eclipse takes place, ideally timed for North Americans to observe in the evening. The Moon's first contact with the Sun takes place at 4:25 PM CDT, and the moment of greatest eclipse is at 5:31 PM. These times are for an observer located in Minneapolis, MN; for other parts of the US, the times will vary slightly. Anyone wishing to view the eclipse should view it through special solar observing filters or by projecting the Sun's image. Don't ever stare directly at the Sun, even with sunglasses, and especially not through a telescope!

To simulate the partial solar eclipse in Starry Night 7 look in the Find Panel (the right sidebar) under "Today's Sky" and click on the listing for "Partial Solar Eclipse". Enjoy SN7!

Will you be viewing the partial solar eclipse today?

Double Stars around Boötes

On a May evening many years ago, I made my first exploration of the night sky. The only star pattern I could recognize was the Big Dipper, but with a star chart in a book, I used that to discover the bright star Arcturus in the constellation Boötes.

The curve of the Big Dipper's handle leads to Arcturus, the brightest star in the kite-shaped constellation of Boötes. Surrounding Boötes is an amazing variety of double stars. Credit: Starry Night software

The trick to learning the constellations is to begin with the stars you know, and use them to identify their neighbors. This same technique, known as "starhopping" is the key to discovering all the wonders hidden amongst the stars.

Start, as I did, with the Big Dipper, high overhead as the sky gets dark at this time of year. The stars that form the Dipper’s handle fall in a gentle arc, and if you project that arc away from the Dipper’s bowl, you come to a bright star. This is Arcturus, the third brightest star in the night sky, and the brightest star in the northern sky. Only Sirius and Canopus, far to the south, are brighter.

Arcturus is bright in our sky for two reasons, first because it is relatively close to us, 38 light years away, and secondly because it is inherently a bright star, much brighter than our Sun. Though larger and brighter, it is a slightly cooler star than our Sun, so appears orange to our eyes.

Although Boötes is supposed to be a ploughman in mythology, its pattern of stars most resembles a kite, with Arcturus marking the bottom of the kite where the tail attaches. Notice the little dots over the second "o" in Boötes: this indicates that the two "o"s are supposed to be pronounced separately, as "bow-oo’-tees," not "boo’-tees."

Once you have identified Boötes, you can use its stars to identify a number of constellations surrounding it. Between it and the Big Dipper are two small constellations, Canes Venatici (the hunting dogs) and Coma Berenices (Bernice's hair). To Boötes left (towards the eastern horizon) is the distinctive keystone of Hercules. Between Hercules and Boötes is Corona Borealis (the northern crown) with Serpens Caput, the head of the serpent, poking up from the south.

Although most stars appear to our unaided eyes as single points of light, anyone with access to binoculars or a telescope soon discovers that nearly half the stars in the sky are either double or multiple stars. Some of these are just accidents of perspective, one star happening to appear in the same line of sight as another, but many are true binary stars: two stars in orbit around each other, similar to the stars which shine on the fictional planet Tatooine in Star Wars.

Every star labeled on this map of Hercules, Boötes, and Ursa Major is a double star, worth exploring with a small telescope. Some, like Mizar in the Dipper’s handle, can be split with the naked eye. A closer look with a telescope shows that this is really a triple star. Others require binoculars or a small telescope. Some of the finest are Cor Caroli in Canes Venatici, Izar (Epsilon) in Boötes, Delta Serpentis, and Rho Herculis.

One of the joys of double star observing is the colour contrasts in some pairs. Others are striking for matching colours and brightness. My favorites are stars of very unequal brightness, which look almost like stars with accompanying planets.

Also marked on this chart are three of the finest deep sky objects: the globular clusters Messier 13 in Hercules and Messier 3 in Canes Venatici, and the Whirlpool Galaxy, Messier 51, tucked just under the end of the Big Dipper’s handle. You will probably need to travel to a dark sky site to spot this galaxy. A six-inch or larger telescope will begin to reveal its spiral arms, including the one that stretches out to its satellite galaxy, NGC 5195.

Starry Night 7: High Resolution Planetary Texture Collection

In recent years, NASA has conducted numerous exploratory missions that provided detailed measurements of the visual appearance and physical characteristics of most of the planets and moons in our solar system. These categories include IR/UV/visible imagery, physical, chemical and geological properties of the celestial bodies.

Measurements range from surface features to physical properties to chemical and geological characteristics. Surface features consist of topography, albedo, roughness and age. Fundamental physical properties include gravity, magnetism and temperature. Chemical and geological quantities comprise elemental composition, mineral distributions, geological maps and crustal thickness. 

Starry Night Pro Plus 7 offers a Planetary Science Texture Compilation with over 100 additional maps. Planetary images and data were derived from the latest datasets available. The original, highest quality sources were used to produce maps with maximum fidelity. Gaps and artifacts in individual datasets were corrected with data from older or alternate sources obtained by other exploratory space missions to produce consistent, high quality images that clearly illustrate the parameters of interest.
 

Earth:
Light Pollution Atlas

Jupiter:
Callisto Color
Callisto Gray
Europa Color
Europa Gray
Ganymede Color
Ganymede Gray
Io Color
Io Geology
Io Gray
Jupiter Color
Shoemaker-Levy

Mars:
Mars Albedo Color
Mars Albedo Gray
Mars Elemental Abundance Set
Mars Geoid
Mars Geological Map
Mars Gravity
Mars Magnetic Field
Mars MDIM
Mars Roughness
Mars Surface Dust Index
Mars Thermal Inertia
Mars Topo Jade
Mars Topo Spectrum
Mars Viking Color
Mars Viking MDIM Merged
Mars Viking Shaded
MOC color
MOC gray

Mercury:
Messenger Color
Messenger Gray
Mercury Messenger Color
Mercury Messenger Gray

Moon:
Clementine Color
Clementine False Color
Clementine Gray
Clementine Iron
Clementine Mineral Ratio
Clementine Optical Maturity
Lunar Gravity
Moon Crustal Thickness
Moon Elemental Abundance Set
Moon Geoid
Moon LROC Gray
Moon Roughness
Moon Temperature
Moon Topo
Moon Illusion Beetle
Moon Illusion Lady
Moon Illusion Lady Reading Book
Moon Illusion Man In Moon
Moon Illusion Rabbit
Moon Illusion St. George

Saturn:
Dione Gray
Enceladus Color
Enceladus Gray
Iapetus Color
Iapetus Gray
Mimas Gray
Phoebe Gray
Rhea Gray
Saturn Bjorn Jonsson
Saturn Hubble
Tethys Gray
Titan Color
Titan Gray
Titan IR
Titan Lakes
Titan Topo
Titan Topo scale

Sun:
Ca II 3933A
FeIX-FeX 171A
FeIX-FeX 171APNG Tiles
FeVII 195A
H-alpha
He II 304A

Venus:
Venus Geoid
Venus Gravity
Venus Magellan Color
Venus Magellan Gray
Venus Topo

Vesta:
Vesta Gray
Vesta Rock Types
Vesta Topo

Starry Night 7: Streaming Data On-Demand

Starry Night Pro Plus 6, when fully installed, occupied more than 10GBs of disk space. The sheer size of it required jumping through lots of technological hoops; from huge downloads to special DVDs, each with their own set of gotchas. This led to a lot of problems, all to deliver every last bit of data, much of which might never be accessed!

With the ever expanding volumes of interesting astronomical data available to the general public, as well as the near ubiquitous availability of high-speed internet access, Starry Night 7 was designed with a new mantra... deliver it on-demand!

Not wanting to sacrifice our tradition of beautifully simulating huge sets of astronomical data, we designed a new, robust system to stream the high-resolution, or rarely accessed data as its needed.

Not only do we now have the ability to deliver you, our users, essentially infinite amounts of astronomical data, but we can now deliver you ONLY that data which you want.

The system, at its base, is quite simple:

Zoom in on a particular piece of the sky, descend onto Mars, or choose one of dozens of available horizon panoramas and Starry Night checks your local hard disk for the necessary files. If they're found, they're loaded into the sky. If not, Starry Night makes a request to our servers to download the necessary data, streaming it to your computer where it will remain for the next time you need it.

This was done in a very limited way in Starry Night 6... only very dim USNO stars were served up by this system.

We have since expanded on that (and will continue to expand on that) to include the AllSkyImage layer and dozens of high-resolution planet surface textures. Horizon Panoramas are next to come along with individual object data, and more and deeper databases.

Some 27 GBs of data rests on our servers, waiting to be streamed! Sit back, zoom in, and enjoy!

Starry Night 7.0.2 and Beyond

Development of Starry Night 7 is proceeding quickly, so I thought I'd take a sec to update everyone on what we're working on.

In the very short term, we'll release Starry Night 7.0.2, mostly to address crashes and other incompatibilities that weren't discovered during beta-testing. We're focusing on issues that make it difficult to use the app, and features that might be broken. I'm taking this week to true up the Equipment list too.

In the weeks to come, we're going to take a solid look at all of the feature requests, suggestions and comments all of you have made, as well as usability issues discovered by our Beta-Testers.

We'll be rewriting the Observation Logging feature, with a particular emphasis on sharing logs... with each other and with other applications. We'll be sure to enable importing of your V6 logs, not to worry!

In addition to improving all of the obvious observing features, we'll be looking at other ways to share experiences with your friends and co-observers, improve add-to and refine our databases, and improve the speed of the application.

Stick with us. Lots of interesting things to come.

Importing SN6 data to SN7 (Or: How I learned to stop worrying and love Sky Data overrides.)

Because of the ubiquity of the internet, the vast volumes of data available to us (and therefore you, the SN user) and new security requirements imposed by operating systems, Starry Night 7 has introduced the idea of a "dynamic" (writeable) Sky Data folder.

In previous versions of SN, the application itself would edit (write to) files in its own Sky Data folder located either in the application package (on OSX) or in the Program Files folder (Windows) and this is now considered very bad behavior.

For that reason, any time SN needs to write/modify a file, we so in a new Sky Data folder located at:

(Windows) \Users\<YourUserName>\AppData\Local\Simulation Curriculum\Starry Night Prefs\

(Note that the AppData folder is often "hidden". A quick Googling can show you how to un-hide it)

(OS X)  /Users/<YourUserName>/Library/Application Support/Simulation Curriculum/Starry Night Prefs/

Note that this is where any "streamed" data will be located too.

While we've made every effort to hide this ugliness from you the user, if for any reason you want to get your hands dirty and edit a file manually, it should first be copied to the same respective path (e.g. /Sky Data/Planet Images) in this new folder, then edited there.

Any file located in this new Sky Data folder should have the effect of overridding the one in our static Sky Data folder.

Now, for the part you've all been waiting for. How do you get all of your Equipment, Distance Spheres, Locations, Preferences, Custom (User) Planets etc. from SN6 to SN7? 

Simply copy the individual files from your old "Prefs" folders into the matching locations in the new "Starry Night Prefs" folder!

Have fun!

Starry Night 7: Motivation, Process, Future

As many of you have noticed, we're back with a brand new version of Starry Night! Rather than list all of the cool new features, I thought I'd take a moment to make clear our motivations for the changes in SN, our process getting to where we are now, and plans for the future.

For many of our loyal, longstanding users, the new user interface is a big change from what they're used to. Our motivation for the change was simple: the interface had gotten to the point where users were spending more time looking at (or looking for!) controls, than looking at the sky. We needed something new, cleaner, less obtrusive.

In our redesign, we followed the general philosophy that the UI should "be there when you need it, disappear when you don't."  The focus should always be on the sky view, never the controls. This for example, is why we moved the Find pane from the left to the right... in general, people read left-to-right. Left is more prominent, so the sky view should always be at the left.

Our move to a "Universal Search" function was similarly motivated. 

We found that over the years, so many of the great new features that we had added were buried under layers of user interface, that not only were they hard to use, but people often never found them in the first place.

With the ability to do a textual search for control items (not just named objects in the night sky), we have opened up a host of existing features to users who didn't even know about them! No longer do you need to know exactly what setting you're looking for, open the Options panel, visually search for it, and click to make a change... simply search for the word (or even a related word) that you're looking for, and you'll probably find it.

While I think we have succeeded in many, many ways, we still have much work to do.

Going forward, we plan on continuing with the idea of "less is more." Not in terms of what you can do (indeed, we are addingfeatures and data) but in what ways you are distracted from what you are doing. Think: more of what you want, less of what you don't.

While we have released it into the wild, we're far from done with it. Starry Night 7 should be thought of as a journey, not a destination.

Photos of Apollo missions

Houston's Lunar and Planetary Institute has made available almost 20,000 otherworldly images taken during Apollo missions (http://www.lpi.usra.edu/resources/apollo/catalog/70mm/ ). Dig deep and you'll find plenty of Instagrammable moments (eg. http://www.lpi.usra.edu/resources/apollo/frame/?AS16-116-18636 )

Can you match the photos with Apollo mission paths in Starry Night?

Orion and His Friends and Enemies

On winter evenings, the sky is filled with bright stars, more than at any other time of the year.

On winter evenings, Orion dominates the sky, surrounded by numerous striking constellations, all decorated with brilliant stars.  Credit: Starry Night Software

Central in the southern sky is the constellation of Orion the Hunter. Along with the Big Dipper, this is probably the most easily recognized constellation, and the starting place for a stargazing adventure.

We apologize to our readers in the southern hemisphere, where it is summer. But even in the south, Orion dominates the sky right now. Turn the chart upside down, and everything we say will still apply.

Orion itself sits astride the celestial equator, half way between north and south celestial poles. This makes Orion an “equal opportunity” constellation, well seen everywhere on Earth except at the poles.

The main figure of Orion is a large rectangle of four bright stars, including two of the brightest stars in the sky, Betelgeuse at upper left and Rigel at bottom right. These four stars represent the shoulders and knees of a might hunter.

The thing that most people notice first is the diagonal line of bright stars right in the middle of the rectangle, which represent the giant’s belt, worn at a jaunty angle. Hanging from his belt are three stars representing his sword.

If you’re located at a dark sky site, you will be able to see more details in Orion. His rather small pointy head is represented by a triangle of stars. His right arm raises a club and his left arm raises something towards Taurus the Bull. Some legends have this as a lion’s skin, others as a shield.

I like to see Orion as a superhero beset by evildoers on all sides, but also with friends and allies.

Taurus, to his upper right, is marked by a bright red star, Aldebaran, in the midst of the cluster of stars known as the Hyades. A bit higher is a second cluster, the Pleiades. Both clusters are easily seen with the naked eye. Orion is shielding himself from the Bull with his lion’s skin.

Below Taurus, to the right of Orion, is a meandering stream of stars which early astronomers saw as the river Eridanus. This river meanders below the southern horizon for most people in the U.S.A., but those in southern Florida and Texas may catch a glimpse of its destination, the first magnitude star Achernar.

Above the horns of Taurus is Auriga the Charioteer, marked by Capella, the sixth brightest star in the night sky. I see him as the cavalry riding to Orion’s rescue.

Above and to the left of Orion is the constellation Gemini, the Twins, with its two bright stars Castor and Pollux. Currently this is where the planet Jupiter is located, outshining all the stars. So which is Castor and which is Pollux? I remember them because Castor is closest to Capella, both starting with a “C,” while Pollux is closest to Procyon, both starting with a “P.”

Orion, like all good hunters, is accompanied by his two hunting dogs, big and small: Canis Major and Canis Minor. “Canis” means “dog,” “major” means large, and “minor” means “small.”

Each dog contains one bright star: Procyon in Canis Minor and Sirius in Canis Major. There is only one brightish star besides Procyon in Canis Minor, making it more like a hot dog than a real dog. Canis Major is more like a real dog, sitting up with a head, body, and two hind feet. Sirius and Procyon are the first and eighth brightest stars in the night sky, and among the nearest to the sun at 8.6 and 11.4 light years distance respectively.

Between the two dogs is a faint constellation with a long name: Monoceros. “Mono” means “one” and “ceros” means “horn,” so Monoceros is a unicorn. Although it lacks any bright stars, it is one of the richest constellations in deep sky objects, because an arm of the Milky Way lies in this direction.

What is beneath Orion’s feet? Usually called Lepus the Hare, I like to think of this as Monty Python’s Killer Rabbit, yet another foe for our hero to vanquish.

Everything I’ve described can be seen with the unaided eye, even in fairly light polluted skies. If you have binoculars or a small telescope, there are incredible riches to be discovered, such as the clouds of glowing gas in Orion and Monoceros, the star clusters of Taurus, Auriga, Monoceros, and Canis Major, and the galaxies of Eridanus.

Venus Shines at its Brightest

This week Venus will be shining at its brightest, low in the southwestern sky just after sunset. Venus’ brightness is the result of geometry.

At 2 p.m. EST on Friday December 6, Venus will be shining at its brightest. Look for it in the southwestern sky just after sunset.  Credit: Starry Night Software

As Venus moves around the Sun, closer to it than the Earth, we see it illuminated from all angles.  This causes it to pass through a series of “phases” similar to the moon. When it is on the far side of the Sun, called “superior conjunction,” it is fully illuminated from our point of view, and we see it as a “full Venus.” It is 100 percent illuminated but far away, only 10 arc seconds in diameter.

When Venus is at “greatest elongation,” farthest from the Sun in our sky, as it was on November 1, we see it as a “half Venus.” When it passes between Earth and the sun, as it will on January 11 2014, called “inferior conjunction,” it is illuminated from behind, just like the new moon.

The brightness we see from Venus depends on two things: its phase and its distance from us.  It should be brightest at its “full” phase, like the Moon, but at that time it is at its furthest from us. At “half” phase, as it was on November 1, only half of it is illuminated, but it is much brighter because it is much closer.

As Venus nears inferior conjunction, its illuminated portion shrinks down to a narrow sliver. This causes it to fade in brightness. But it is also getting closer to us, which makes it brighten.  This week, these two factors balance out, and we will see Venus at its very brightest. It is neither “half Venus” (50 percent illuminated, 25 arc seconds in diameter) or “new Venus” (0 percent illuminated, 60 arc seconds in diameter), but somewhere in between. In fact it is 26 percent illuminated and 41 arc seconds in diameter. This is the “Goldilocks point” when distance and  phase combine to produce the greatest brightness.

This week Venus will shine with a brightness of –4.9 magnitude, on the upside-down brightness scale that astronomers use. It is based on the brightest stars being magnitude 1 and the faintest stars visible being magnitude 6. Thus the brighter the object, the smaller its magnitude number. 

Astronomers extended this scale into the negative for really bright objects.  The brightest star in the night sky, Sirius, is magnitude –1.4. The full moon is –12.7 and the sun is –26.8. So Venus this week will be considerably brighter than Sirius, but nowhere near as bright as the moon. It is bright enough to cast shadows, when observed on a moonless night from a dark location.

Even though Venus is the brightest object in the night sky other than the moon, surprisingly few people have seen it in its current apparition. That’s because at this time of year the ecliptic, the path of the planets across the sky, makes a very shallow angle with the horizon in the northern hemisphere. Although Venus is very bright, it is also very low in the sky, so is often blocked by clouds or buildings.

This week, find yourself a location with a low southwestern horizon and look for Venus. Watch it as it slowly sets, and see if you can see it change color from white to orange to red as it nears the horizon, just as the sun and moon do.

Did you know that you can see Venus in daylight? The best time to look for it will be on Thursday this week. Look for the narrow crescent moon in the afternoon sky above and to the left of the sun. Use that to locate Venus, just below the moon. You may need binoculars to first spot it, but once you know where it is relative to the moon, it’s very easy to see.