Located about 100 miles west of London, the city of Bath is known for the ancient Roman Baths that attract 1 million visitors each year. One half mile west from the baths is the Herschel Museum of Astronomy, the 18th century residence of William Herschel. As an observational astronomer, William Herschel tends to get overlooked by the great theorists such as Issac Newton. Nonetheless, the work Herschel did in Bath greatly expanded our knowledge of the universe and remains topical in astronomy research.
In contemporary parlance, Herschel was a career changer. Originally a musician by trade, Herschel took an interest in astronomy in 1773 at the age of 35. Herschel was a self-made man. He had no formal training in astronomy and taught himself the art of telescope making. What had perked Herschel’s interest in astronomy was a book on musical mathematics called Harmonics by Robert Smith. Herschel enjoyed the book so much he sought out other books by Smith and found one titled Opticks. This book, along with Astronomy by James Ferguson, formed the basis of Herschel’s training in the field. Herschel remained a music teacher during the the day and astronomer at night. In his endeavors he was joined by his sister, Caroline Herschel, who became his lifelong assistant.
Above: Herschel’s Symphony No. 8 in C minor by London Mozart Players. Written in 1761, it is one of 24 symphonies composed by William Herschel.
Herschel was unable to buy a telescope suitable for his ambitions. As a result, along with his sister Caroline, he took to the task of making his own telescopes. Astronomers today do not need to do this obviously, but this is similar to the manner many astronomers write their own computer codes for their work. This type of specialized software is not available at a store in your local shopping mall. Over his lifetime, Herschel would grind and polish hundreds of mirrors, some of which he sold to help fund his work.
Herschel’s primary goal was quite formidable, to conduct an all-sky survey. Motorized drives to track objects as they moved in the night sky were not available in the 19th century, so Herschel would observe at a fixed angle on the meridian and logged objects as they crossed the field of view. The next evening, Herschel would lower or raise the telescope to a different angle for complete coverage of the night sky. This effort resulted in the publication of the Catalogue of Nebulae and Clusters of Stars (CN) in 1786, the forerunner of the New General Catalouge (NGC). Along the way, Herschel would make quite a few interesting discoveries.
On the night of March 13, 1781, from his residence in Bath, Herschel observed in his 6-inch telescope what he thought was a comet. Herschel noted:
“On Tuesday, the 13th of March, 1781, between ten and eleven in the evening, while I was examining the small stars in the neighborhood of H Geminorum, I perceived one that appeared visibly larger than the rest: being struck with its uncommon magnitude, I compared it to H Geminorum and the small star in the quartile between Auriga and Gemini, and finding it so much larger than either of them, suspected it to be a comet.”
Measurements of the orbit of this object revealed it to be not a comet, but a planet, the first planet discovered since the ancient astronomers categorized the five naked eye planets of Mercury, Venus, Mars, Jupiter, and Saturn. Below is an image of how the night sky appeared in Bath as Herschel made his first observation of this planet.
Herschel wanted to call this planet Georgium Sidus (The Georgian Star) to honor King George III. Others sought a less English-centric name. Uranus was proposed as in Greek mythology, Uranus is the father of Saturn. It was not until 1850 that the planet was officially designated as Uranus. As Uranus is twice the distance (1,783,939,400 miles or 2,870,972,200 km) to the Sun as Saturn, this discovery doubled the size of the known Solar System. It takes 84 years for Uranus to orbit the Sun. Thus, Uranus has only made 2.8 revolutions of the Sun since its discovery. In 1986, Voyager II would become the only spacecraft to date to pay a visit to Uranus. A view of Uranus from Voyager II is below:
Uranus’ South Pole was facing Voyager II as it is inclined 98 degrees compared to Earth’s 23.5 degree axial tilt. If Earth had the same axial tilt as Uranus, the Northern Hemisphere would face the Sun in June while the entire Southern Hemisphere would be in darkness. The situation would be reversed in December. When Voyager II flew past Uranus, the Northern Hemisphere was shrouded in darkness. If NASA’s plans to send an orbiter around Uranus comes to fruition in the 2030’s, the Northern Hemisphere would then be visible.
This discovery was a game changer for Herschel. King George III, as the Revolutionary War raged in the American colonies, provided Herschel with a salary to pursue astronomy on a full-time basis. This would launch Herschel on a decade of discovery.
In 1784, Herschel published On the Remarkable Appearances at the Polar Regions on the Planet Mars. This paper presented the results of observations taken of Mars from 1777 to 1783. A few of Herschel’s drawings of Mars is below:
Among the conclusions Herschel came to from these observations are:
The axial tilt of Mars is 280 42′, reasonably close to the now established value of 25 degrees.
The length of the Martian day as 24 hours, 39 minutes, and 21 seconds. This measurement was off by only 2 minutes.
The luminous areas at the polar regions were ice caps, which like Earth, would vary in size on a seasonal basis. Today, we know the northern ice cap has a permanent layer of water ice. The southern ice cap has a permanent top layer of 8 meters of carbon dioxide ice and a much larger layer of water ice below. The seasonal variations of the ice caps are due to the freezing and evaporation of carbon dioxide ice.
Herschel concluded his paper by stating, “And the planet has a considerable but moderate atmosphere, so that its inhabitants probably enjoy a situation in many respects similar to our own.” Ok, this one didn’t quite pan out as we know Mars’ mostly carbon dioxide atmosphere is much thinner than Earth’s and life does not exist on the surface. However, Mars atmosphere in its ancient past must have been warmer and more substantial for water to have been present on the surface, of which the evidence is now pretty conclusive. The search for life in Mars’ past and microbial life in the Martian sub-surface, which still has water, is a major component in NASA’s Mars Exploration Program.
While several rovers and orbiters have provided thousands of high resolution images of Mars, Earth bound telescopes still acquire key data on Mars past and present:
Herschel would also discover two moons of both Saturn and Uranus.
The Uranus moons were discovered on the same day in 1787 and were named Titania and Oberon. Both moons were imaged by Voyager II on its flyby of Uranus. Titania featured fault valleys as long as 1,500 km and Oberon has a mountain 4 miles high.
Two years later, Herschel would discover the Saturn moons Mimas and Enceladus. Both these moons have been imaged by the Cassini orbiter mission. Mimas features a large impact crater that has given it the nickname “Death Star”.
The crater has been named in Herschel’s honor. The crater itself is 140 km (88 miles) wide and the outer walls are 5 km high with a central peak 6 km high. An impact just a bit larger would have most likely destroyed Mimas. As interesting as this is, it is Enceladus that has proven to be one of the biggest surprises of the Cassini mission.
Only 500 km wide, Enceladus is very bright as it reflects almost 100% of the sunlight it receives. Thought to be too small for geologic activity, Enceladus provided an unexpected finding when Cassini imaged geysers spraying ice and water vapor into space. Further gravity analysis indicates an ocean 10 km deep underneath a ice shell 30-40 km deep. Recently, it has been determined the geysers are more akin to curtain eruptions seen in volcanic activity in Hawaii and Iceland. Still, this water is thought to be at least 194 degrees Fahrenheit at the ocean floor, the heat generated by gravitational flexing from Saturn. Where there is heat and water, there may be life. Cassini has flown through the geysers but its instrument package was not specifically designed for this task. As such, Enceladus is a priority for NASA exploration in the next decade. Unlike the subsurface ocean of Europa, the ocean of Enceladus could be sampled without having to bore down through several kilometers of ice.
As impressive as Herschel’s Solar System discoveries were, the task to complete an all-sky survey meant he studied deep space objects moreso than planets and their satellites. Herschel would discover numerous nebulae and binary stars that prior to his telescope, were not resolvable. By 1785, with the salary granted by King George III, Herschel had moved from Bath to London and was using a 19-inch aperture telescope to map the Milky Way. The results were published as On the Construction of the Heavens.
The bright spot in the center is the Sun. Herschel was operating under the handicap of observing in visible light only, which is extinguished by the interstellar medium. This gave the illusion the Sun was located in the center of the Milky Way as the interstellar medium dampened optical light in all directions equally. It is like trying to map trees in a foggy forest. There may be more trees in one direction than the other, but the fog cuts down on your vision at equal depths in all directions. In fact, it was not until the 1920’s when Harlow Shapley determined the Sun was located in a spiral arm of the Milky Way and not in the center was this problem resolved. For astronomers to obtain a comprehensive view of the universe, the entire electromagnetic spectrum had to be employed. And it was Herschel who provided the first step in that direction.
In 1800, Herschel was measuring the temperatures of the different colors of sunlight separated by a prism. As Herschel took temperatures from the violet end of the spectrum to the red he discovered an increase in temperature as the thermometer was moved towards the red. Finally, the thermometer was placed just beyond the red light, and the temperature increased even more. It was apparent the Sun was emitting some form of radiation beyond the furthest end of the visible spectrum. More experiments revealed this invisible radiation had the same properties as visible light, it could be reflected and refracted. Herschel published this result in the paper titled, Experiments on the Refrangibility of the Invisible Rays of the Sun. Herschel referred to this radiation as calorific (heat) rays, today we call it infrared light.
Optical light is just a small part of the electromagnetic spectrum. Among the other parts we are unable to detect with our eyes, we can detect radio waves with radio receivers, ultraviolet waves with our skin when we get sunburn, and x-rays with film when we go to the doctor. Those forms of radiation only differ from light in the size of their respective wavelengths and consequently, their energy. Infrared is used for remote control and night vision technology. Most of the heat we feel in our day-to-day activities is the result of infrared light and our bodies emit infrared radiation in the form of body heat which is detected in night vision sensors.
Planets radiate mostly in the infrared, as do cool galactic gas clouds. Certain wavelengths of infrared radiation has the ability to pass through dust clouds. Thus, infrared observations can peer into dusty regions in space and see what lies behind the shroud of dust. As a result, infrared astronomy is used for planetary observations, to detect protostars inside of nebulae, and to peer into the galactic center behind the wall of interstellar dust. In other words, the form of radiation Herschel discovered is now used to better understand the very objects Herschel observed.
The video below is a montage of 2.5 million images of the Milky Way taken by the Spitzer Infrared Space Telescope. As certain wavelengths of infrared are not absorbed by the interstellar medium as optical light is, the Spitzer images provide us with the true shape of our home galaxy including the central bulge that contains a massive black hole.
The Spitzer GLIMPSE360 website has an interactive where you can explore different regions of the Milky Way or select objects to view. The Milky Way is not the only region that can be explored in infrared. In 2014, the Keck Observatory imaged Uranus with infrared.
Images of Uranus, such as the ones taken by Voyager, tend to reveal a featureless planetary disk. However, the Keck infrared image revealed storm activity to an extent not seen before on Uranus. This might be indicative of an internal heat source that was not thought to exist previously on the gas giant. Astronomers will need to revise current theories on the interior of Uranus as a result of this work.
As one would expect, many honors have been accorded upon the Herschel name. This would include the 3.5 meter infrared Herschel Space Observatory and the 4.2 meter William Herschel Telescope in the Canary Islands. However, the highest honor we can bestow upon William Herschel is the continued exploration of the celestial bodies he discovered, using the infrared radiation that he also discovered.
*Image on top of post, Sir William Herschel, by Lemuel Francis Abbott, oil on canvas, 1785, © National Portrait Gallery, London, Creative Commons License.