astronomy@iowa

At 11:59 UT¹ the Sun reached its most northerly point above the equator and marked the beginning of summer in the northern hemisphere. From the Latin name for the Sun (Sol) and sistere (meaning to stand still), solstice refers to the two times during the year when the Sun’s annual apparent motion around Earth’s sky reaches the most northerly and southerly extremes. The summer solstice—sometimes incorrectly referred to as the longest day of the year—is not any longer than other days of the year. The fact that the Sun reaches its most northerly point at the summer solstice simply means that today is the date that has the most daylight hours and the shortest night. And while we use this event to mark the beginning of summer in the northern hemisphere today, that has not always been the case.

Once referred to as midsummer (see “A Midsummer Night’s Dream“), the summer solstice once marked the middle of summer while the winter solstice marked the middle of winter and the two equinoces (“equal nights”) marked the middle of spring and autumn respectively. In some old European (such as ancient Irish) and East Asian cultures, the actual seasonal changes were related more to what are today called cross-quarter days. Based on modern calendars, these pagan celebrations fell at the beginning of November (Samhain), February (Imbolc), May (Beltane) and August (Lugnasadh) and are still observed today. Traditionally the new day began at sunset so many of these festivals span the eve and next morning.

Late October/Early November: Halloween, All Saints Day, All Souls Day
Early February: St. Brigids Day, Groundhog Day, Candlemas
Late April/Early May: Walpurgis Night, May Day
Early August: Lugnasadh, Lammas

The holiday that may be least familiar to those living in North America is Lammas. The Loaf-mass was a celebration of the first wheat harvest of the year and it was traditional to bring a loaf to church that was made from that first-harvest wheat.

While we often associate summer with the warmest season, it is wrong to assume the warmth of summer is due to our proximity to the Sun. Earth’s orbit is slightly eccentric (Kepler’s first law of planetary motion notes that planets orbit the Sun along elliptical paths), but distance is not the contributing factor to summer warmth. In fact, Earth is at its closest point to the Sun (perihelion) in early January and at its farthest point (aphelion) in early July! It is the inclination of Earth’s axis of rotation and the angle of sunlight striking Earth (see figure) that causes both the variation in daylight hours and efficiency of solar heating responsible for seasonal variations.

¹ UT refers to Universal Time and is the astronomical standard of time based on the mean solar time at the Greenwich meridian, which marks 0° longitude. To convert to the local time zone, simply add or subtract (taking daylight savings time into consideration!) the number of time zones between your location and the Greenwich meridian. For example, the Central Time Zone is 6 hours west of Greenwich so Central Standard Time  is UT – 6 hours while Central Daylight Time is UT – 5 hours. Hence, the summer solstice occurred at 11:59 p.m. – 5 hrs or 6:59 p.m. on June 20.

Image in link courtesy of Przemyslaw “Blueshade” Idzkiewicz. This work is licensed under the Creative Commons Attribution-ShareAlike 2.0 License.

Two years ago the International Astronomical Union (IAU) angered a lot of people when the Solar System’s ninth planet Pluto was demoted and designated a “dwarf planet” along with the asteroid Ceres and the recently discovered trans-neptunian object Eris. Well, the IAU has now come up with an official designation for Pluto and Eris that is almost guaranteed to upset people all over again. While some may think it’s a silly exercise to spend so much energy coming up with classification schemes (years ago the physicist Enrico Fermi complained about the number of sub-atomic particles saying “If I could remember the names of all these particles, I’d be a botanist.”), it is important for scientists to make classifications of the objects they study. The Greeks used the term “planetes astrum” to describe the wandering stars—as opposed to the “fixed” stars—but that definition did not really distinguish the actual planets from the Moon and Sun. But “Plutoid?”

The IAU Committee on Small Body Nomenclature defines a plutoid as “celestial bodies in orbit around the Sun at a semimajor axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit.” For naming purposes, any body that meets these criteria and have an absolute magnitude brighter than H = +1 where H is the magnitude of the planet, asteroid, comet, etc. at one Astronomical Unit from the Sun.

In the above image, Pluto and its three moons are shown on the left while Eris is on the right.

Image Credit: IAU, NASA/ESA Hubble Space Telescope, H. Weaver (JHU/APL), A. Stern (SwRI), the HST Pluto Companion Search Team and M. Brown.

The following Oo-Ed piece by physicist Brian Greene appeared in the New York Times today:

Put a Little Science in Your Life