The Cosmic Distance Ladder, part 1…

For the past few months, I’ve been spending a lot of time in my position as Manager of the UNH Observatory, in helping to prepare for the 2012 New England Fall Astronomy Festival. The event, lovingly known as NEFAF, is a family-friendly astronomy-related event that will be hosted by the UNH Physics Department in partnership with the New Hampshire Astronomical Society. As you can imagine, this is quite an undertaking, but in an incredibly exciting turn of events, we just found out that Dr. Alex Filippenko, noted astronomer from UC-Berkeley and member of the research team that won the 2011 Nobel Prize in Physics, will be giving the keynote talk at NEFAF 2012! In addition to being a highly acclaimed professor, Filippenko is also the co-author of an extremely popular astronomy textbook and a frequent contributor to the documentary series The Universe on The History Channel.

Dr. Alex Filippenko, the newly announced keynote speaker for the 2012 New England Fall Astronomy Festival to be held at the UNH Observatory.

That extremely exciting news has inspired me to do a couple of posts about the expansion of the universe, the area of research that Dr. Filippenko works on. But before we can really get into talking about that, we need to cover a very basic aspect of astronomy, but something that most non-astronomers don’t really know about. I was at a public session at the Observatory this weekend when a guest who had never studied astronomy before asked me what she thought might be an “ignorant” question: she wanted to know how exactly astronomers knew the distances to objects in space. This is by no means an ignorant question, in fact it’s a very fundamental and very involved question that really gets at the very nature of astronomy.

Astronomy by definition is an observational science. Unlike many other scientific disciplines, astronomers can’t really do experiments in a laboratory (although some do). But the stereotypical astronomer can’t throw his subject (a star or galaxy) on a lab bench and dissect it or set up an experiment to test it, so astronomers need to observe and record data. Okay, so we observe light, that tells us what something looks like, where it is, and how bright it is. Big deal, is that really that helpful scientifically. Well, not really. So we have to come up with ways to get more information from observing the light. The main way we do that is by breaking the light up in a spectrometer, an instrument that breaks light down into a spectrum of colors. This breakdown of light can reveal an abundance of new information including what the object is made up of, how hot it is, how fast and in what direction it’s moving, how old it is, and more.

The question of how astronomers calculate the distance to an astronomical object varies depending on how far away the object is. Because most of these techniques only work up to a certain distance, there is actually a progression of different approaches that astronomers use to measure distance to celestial objects. This list of methods of measuring astronomical distances is known as the Cosmic Distance Ladder (or less poetically, the extragalactic distance scale).

A graphical representation of the distance-measuring techniques that make up the Cosmic Distance Ladder. “1 A.U.” is 1 astronomical unit or approximately 93 million miles, the distance from the Earth to the Sun. A “pc” is a parsec, equal to 3.2 lightyears (206,265 A.U.) or about 20 trillion miles. “Mpc” stands for “Megaparsec” or millions of parsecs. Credit: University of Rochester

In our solar system

The first step in our exploration of the universe was to our own celestial neighborhood. The first step was precise measurement of the size scale of the solar system, which started with the determination of the distance between the Earth and the Sun. As I’ve explained before, this measurement was originally calculated via observations of transits of the planet Venus across the disk of the Sun. Early on in the 20th century, observations of asteroids also played an important role in this measurement. But today the distance from the Earth to the Sun, defined as 1 Astronomical Unit or “AU”, is measured with high precision using radar ranging. By bouncing a radar beam off another planet, usually Venus, and measuring the time that beam takes to return to Earth, scientists can very accurately determine the difference in the size of the two planets’ orbits. Using that difference and the ratio of the two orbital sizes, we can very easily calculate the distance the Earth must be from the Sun. We use a similar process even closer to home. During the Apollo missions of the late 1960s-early 1970s, astronauts deployed the lunar laser ranging experiments, arrays of mirrors that allowed scientists to measure the distance to Earth’s only natural satellite with extreme precision using lasers. This radar ranging is how we’ve calculated the distance to most of the objects in our solar system. More recently, we can also use spacecraft in orbit around other planets as a tape measure by measuring the time it takes for a signal to travel from the spacecraft to its controllers on Earth.

Another  way we can get measure the distance to the planets and to nearby stars is a phenomenon known as stellar parallax. This method is less accurate than radar ranging for planets, but is very good for stars in our local stellar neighborhood. Parallax is something you experience almost every day. Hold your thumb up at arm’s length. Close one eye, then open that one and close the other. Notice how your thumb appears to shift with respect to the objects far off in the distance? That’s parallax! Astronomers take measurements of a planet or star a two points in Earth’s orbit (6 months apart) and measure the angular shift of an object with respect to the background stars between those two measurements. Then, because we know the distance the Earth is from the Sun, we can use some basic geometry to calculate the distance to that star or planet in the foreground.

This diagram shows how parallax is used to find the distance to planets in our solar system and nearby stars. Scientists make two observations 6 months apart, measuring the angle that an object (the red dot) makes with regard to the background stars between the two observations. Then using the distance from the Earth to the Sun (1 A.U.) and some simple geometry, the distance to the object (d) can be calculated. Credit: Hyperphysics

It was using parallax, that Italian astronomer Giovanni Domenico Cassini was able to roughly calculate the distance to Mars in 1672. His calculation was a little bit off though, because instead of taking two measurements 6 months apart, he sent his colleague to Cayenne, French Guiana (on the northern coast of South America) to make observations while he stayed in Paris. Then Cassini could make the same parallax calculation using the known distance between the two observation points (~4400 miles) instead of the distance from the Sun to the Earth. This single direct measurement of the distance to Mars, which is now easily calculated and heavily used by missions such as Curiosity, actually allowed for the calculation of the distances to all the planets. Since geometry and Kepler’s Laws governed the basic ratios the Sun-planet distances, you only needed to measure one Earth-planet distance to be able to easily calculate them all. This major contribution and several others in planetary science (including the discovery of four of Saturn’s moons and joint discovery of Jupiter’s Great Red Spot) prompted NASA to name the Saturn-bound spacecraft mission after the him.

Giovanni Domenico Cassini

[To be continued…]


Related Posts:


Can Venus help us find exoplanets?…

This post is in response to loyal reader Jarman Day-Bohn’s question, which he left in a comment on the post “Today is transit day….”. Jarman asked:

How much do you think this [transit of Venus] will contribute to the current research experts are performing toward the study of possible earth-like planets out there? I know they were heavily using a technique measuring how much of a star’s light is blocked out by a planet to judge its size and other factors. Will this in any way help that process?

Great question Jarman, thanks for asking! Let’s look into that a little bit more.

But before we get too far into that, let’s think a little more about transits. Whether or not a transit occurs is all based on perspective. From Earth, only Mercury and Venus are interior planets- planets orbiting closer to the Sun- so they’re the only two that can we can see transit the Sun’s disk. If you were on Mars though, you could conceivably see Mercury, Venus, and Earth transits. And if you were on Pluto you could, in theory, see all eight planets transit the Sun. Remember though, as you get further away, even though more planets can be seen transitting the Sun from your perspective, you’re also getting further away, meaning the Sun is going to look smaller and smaller to you, as are the transitting planets. The Sun is only 93 million miles from the Earth (that’s really close astronomically speaking), so the enormous Sun, which is 1 million times larger in volume than the Earth, takes up a relatively large portion of the sky (~0.5 degrees). As you move further away from the Sun, its angular size in the sky will shrink. By the time you got to Pluto, which is 3.67 billion miles from the Sun, but still close astronomically speaking, our local star would look like a bright speck only ~0.01 degrees (50 times smaller than in the sky on Earth); probably something similar to the artist’s depiction below.

This artist’s depiction shows what the Sun might look like from an object, like Pluto, that’s in the solar system’s Kuiper Belt. Notice how the relatively close Sun differs from the background stars. Credit: NASA/JPL-Caltech/T. Pyle (SSC)

Now as you travel further from the Sun, let’s say to a planet orbiting another star, and look at our Sun, you could still in theory see all eight planets (and Pluto) transit the Sun, but now you’re trillions of miles from the Sun which is now just point of light in the sky, the same way other stars appear to us in the nighttime sky. The really astounding thing about looking at the other stars in our galaxy (Note: every star you see in the nighttime sky is in the Milky Way, we can’t resolve single stars in other galaxies.) is that they are so incredibly far away that no matter how large a telescope we use, we can never see the disk of the star like we can with the Sun, it just won’t have a large enough angular size. Now matter what, even through the Hubble Space Telescope, stars look like pinpricks of light that astronomers call “point sources“. Which makes actually seeing a planet transit a star, like we can see with Venus, impossible. I’ll remind you that in a previous post entitled “Baseballs, not umbrellas…” I explained the continuing search for exoplanets and covered all three of the main techniques which scientists employ to find these elusive celestial bodies around our galaxy. As Jarman indicated, the most successful and commonly used method of detection is the “transit method”. This is the method that NASA’s Kepler mission has already used to find the first Earth-like rocky exoplanet. However, since I just explained that we can’t actually see the exoplanet transitting the distant star, the only way we can detect the transit is by recording the change in light we see as the exoplanet crosses in front of the star. But again, we don’t actually “see” the transit happen, we just observe the dip in the brightness given off by the star. Similarly, if you were on a ship off the shore and someone walked in front of the lighthouse beacon, you wouldn’t be able to see the person, but you might be able to record the drop in brightness as they walked by.

This artist’s idea of NASA’s Kepler mission looking for exoplanets illustrates the main technique that scientists hope to use to find planets orbiting other stars- called the transit method- but no matter the size of the telescope we can’t actually see the exoplanets transitting the disk of the star. Credit:

But now let’s get back to Jarman’s actual question: can a transit in our solar system, like that of Venus, help scientists to find planets transitting other stars? It might. The transit of Venus is a well-documented and well-understood phenomenon, which scientists have been able to accurately predict and observe at least 6 times in the last 400 years. As I explained in “Looking to launch and preparing for transit…“, the transit of Venus helped us to determine the size of our solar system. And since we’ve actually been able to explore our solar system and have a very good grasp of the size of Venus and the Sun and the distances between the Earth and each of them, we can use the transit of Venus as a calibration tool in our search for extrasolar planets. For instance, scientist can say a planet like Venus, which is so big, transitting in front of a star like the Sun, which is so big, would cause a drop in brightness of this much at this distance. Then we can scale that distance out to other stars and we get some idea of what we need to look for in our search for exoplanets.

So Jarman, there’s your answer: while the spectacular crossing of Venus may not lead to groundbreaking new methods to find exoplanets, it does give us a rare opportunity to view a transit (that involves objects we know a lot about) and use that as a reference point as we continue our search!

Thanks again to Jarman for posing this question and if you, like Jarman, have a question that you’d like to have answered, please leave a comment or use the “Contact astroian” tab at the top of the page to send me an email!


Related Posts:

A glimpse of the transit…

Here’s a screen capture from the live NASA webcast of  visible (white) light image of the surface of the Sun. That black spot at about 3 o’clock is Venus. The other small black features are sunspots on the surface of the Sun. Some sunspots can be larger than the Earth. Credit: NASA

Hey everybody! So I hope all of you are watching the NASA live feed of the transit of Venus and seeing cool things like above.

So here is a mid-transit update of some very cool transit things.

First off, you should check out a very cool website that allows you to make your own compilation of solar images or videos from a multitude of satellites and data from NASA, NOAA, ESA, and more!

Here’s another great shot in the extreme ultraviolet (EUV) from NASA’s Solar Dynamics Observatory (SDO). Again you can see the dark spot, Venus, about halfway across the Sun’s disk. You’ll notice that the EUV allows you to see a lot different detail than the visible light image above. Here we can see the solar corona, or atmosphere, as well as flux tubes (hot plasma trapped along magnetic field lines). That extremely bright region just below Venus with the massive flux tubes flowing out of it is associated with the sunspot you see in the same location on the visible light image above. Sunspots are caused by magnetic field lines on the surface of the Sun that cause a drop in surface temperature (from ~6000 K to only ~3000 K). That temperature drop causes the cooler region to seem dark compared to the rest of the Sun’s surface– hence, sunspots.

An extreme ultraviolet (EUV) image from NASA’s Solar Dynamics Observatory (SDO) of Venus transitting the Sun’s disk. Credit: NASA

And just in case that’s not enough, here’s a link to a movie of Venus’s progression during the transit from SDO, care of @Camilla_SDO.


Related Posts:

Today is transit day…

Hello friends, don’t forget that this evening starting around 6pm EST you’ll be able to see Venus transiting the disk of the Sun, a phenomenon that won’t occur again until 2117!!

Remember, DO NOT LOOK AT THE SUN DIRECTLY WITHOUT PROPER PROTECTION! For safe tips on how to view the transit, click here. Or to be super safe (and mobile) you can check out NASA’s live webcast of the transit from the Keck Observatories at Mauna Kea, Hawaii. That broadcast will feature UNH alumnus and creator Dr. C. Alex Young of NASA Goddard Space Flight Center.

For more information about the transit, you can visit NASA’s official transit webpage,, or

Also, to help you get super pumped about the transit, here are some fun tidbits.

Here’s a link to my friend and fellow grad student, Mark Zastrow from Boston University’s Department of Astronomy, detailing the amazing and harrowing tale of 18th century French astronomer Guillaume Le Gentil‘s trek to observe the pair of Venus transits in the 1760s: The Worst Observing Run Ever

And to prove that Venus is actually going to cross the disk of the Sun, here’s a video from NASA’s SOlar and Heliospheric Observatory (SOHO) satellite. The video is of a series of coronagraph images. Coronagraphs image the corona, or atmosphere of the Sun, and associated flares and coronal mass ejections by blocking out the extremely bright disk of the sun. You’ll notice the big empty blue spot in the middle of the picture, that’s the blocked out part and that white ring indicates the actual outline of the Sun’s circumference. The Sun’s really bright, so you need to block out a lot more than just the disk. You’ll see some big blobs of particles and plasma coming off the Sun, that’s normal solar activity. And finally you’ll see that bright image approaching the Sun from the left, that’s Venus, reflecting the incredibly bright light from the Sun.

Hope you like all that. Enjoy the transit! And if you have any cool transit-viewing pics or experiences, feel free to share them in the comments!


Related Posts:

Looking to launch and preparing for transit…

As many of you probably heard, SpaceX’s launch of its Falcon 9 and Dragon capsule to the International Space Station (ISS) that was slated for early Saturday morning was aborted at literally the last second before launch. The abort was triggered by readings of excess pressure in one of the rocket’s engines. The problem was ultimately traced back to a faulty valve on one of the rocket engines which was replaced on Sunday. Now with everything supposedly right as rain, SpaceX looks forward to launching at the opening of the next possible launch window, which opens at 3:44am on Tuesday. As you may remember, back in December 2010, SpaceX became the first ever private company to launch into space; now it looks into sweetening the deal as it’s slated to become the first private company to dock with the ISS. This is a VERY good thing for the American space program. Despite flurries of protest and new ideas from Congress to limit the competition in space commercialization, SpaceX is doing exactly what President Obama hoped companies would do when he announced the space commercialization initiative back in 2008– beating the rest of the competition to the punch. Not that Obama is specifically backing or rooting for SpaceX, but this is exactly what the President wanted, competition fostering and driving innovation and accelerated success. I’ll have to wholeheartedly disagree with Congressmen who argue that competition breeds lackluster performance and unsafe equipment. Let’s face it, when going into space, there’s an inherent level of risk. Even NASA, the be-all, end-all of space-faring lost two (Columbia and Challenger) out of its five Space Shuttles, so it happens. But let’s examine this: Orbital Sciences, another company vying for NASA launch contracts, has already launched two NASA-funded missions (nearly $700 million) into the Pacific Ocean. Now, using the Congressional argument, we would have been locked into using Orbital Sciences and SpaceX would not have gotten federal subsidies or contracts to help get it to where it is today. Now to be honest, Orbital Sciences and SpaceX are trying to work on two different goals at the moment: Orbital Sciences is set on launching new satellites into space and SpaceX is focused on transporting crews and cargo, but you can see my point. If anything, competition forces greater concern over safety and ensuring success and greatly reduces the probability of project delays and going over budget. And if you don’t think that’s true, just look at how careful SpaceX was this weekend. As President Hoover once said, “Competition is not only the basis of protection to the consumer, but is the incentive to progress.”

Ultraviolet image of Venus’ clouds as seen by the Pioneer Venus Orbiter on February  5, 1979. Credit:

Switching gears a bit, you may or may not have heard, but a once-in-a-lifetime astronomical event is happening on the evening of  Tuesday, June 5, 2012: Venus will be transiting the Sun. What does that mean? Well what that means is that we (Earthlings) will be able to see Venus on the disk of the Sun. You may say to yourself: why is this so special, doesn’t Venus go around the Sun every year? Well yes, you’re right, Venus orbits the Sun once every 225 Earth days, but Venus’ orbit doesn’t exactly lie in the same plane as the rest of the planets– it’s off by what might seem a slight 3°. But since distances are so large in space– Earth is a whopping 93,000,000,000,000 miles from the Sun– that small angle means that Venus only crosses the line of sight between the Earth and the Sun twice (in events separated by eight years) every century. The last transit of Venus was back in 2004 (imaged below) and it won’t happen again until 2117. Now if you’d like to find out more about the transit, you can visit, they’ve got pretty much everything you need to know, including what the transit is, where and how you can see it, a short video summary of the event and why it’s important, and even a recipe for a nice cosmic cocktail to enjoy responsibly while you view the transit! You can also check out or NASA’s official page, which have lots of stuff including information about NASA’s planned live feed of the transit from the Keck telescopes in Hawaii and maps of transit events in your area!

The entire June 2004 transit of Venus is captured in a composite photograph composed of 11 separate images taken at 30 minute intervals. Photographs by Fred Espenak,

Jeremiah Horrocks, an English astronomer, predicted the first ever observed transit of Venus back in 1639– contradicting the great Johannes Kepler, who said that Venus would miss transiting the Sun– and then observed it using a telescope. In fact, the transit of Venus is very important historically– it’s one of the ways we calculated the size of our solar system. You see, when you have two observers watching the transit from two locations on Earth, each sees a distinct path (red and blue below) of Venus across the Sun.  The slight difference in time that Venus takes, moving from edge to edge, can mathematically unlock the distance from Earth to the Sun, and thus the size of our solar system. In fact, after realizing this, the great English astronomer Edmond Halley (of comet fame) greatly encouraged countries to send expeditions around the globe to time future transits of Venus across the sun. Explorers and scientists faced great peril and set out all over the world for the transits of the 17th (1761 and 1769) and 18th (1874 and 1882) centuries.

Two observers from different locations on Earth will see Venus trace different paths across the Sun. The difference in time of the transits between the two paths can be used to calculate the distance from the Earth to the Sun and thusly the size of the solar system. Credit:


Related Posts:

%d bloggers like this: