For many among us, the time is simply a number that comes up constantly throughout everyday life. ’What time is it?’ you might find yourself asking, or ‘I’ll meet you at a quarter past eight.’ But what does time really mean? It is much more than just an arbitrary number that an old man with a long beard came up with thousands of years ago. To truly understand time, and how it works, one must look to the stars, for the basis of the mysterious unit that is time lies in astronomy.

Time is more than just what clocks measure. A dictionary definition is that time is a system of sequential relations between any one event and any other event, in the past, present, or future. This makes sense. Time is the duration between when you put the flobberworm mucus in your potion, and when you need to add the crushed snake fangs. Time is the interval between when you throw the quaffle, and when the Keeper stops it. From a scientific standpoint, time is a physical quantity used to define other quantities. For example, velocity is the distance one travels in a certain amount of time. But once we get to astronomy, time starts to become slightly more complicated. There are things such as spacetime, with time being the fourth dimension in the universe. Time is visualised as a surface that can warp, changing our perception of time when approaching the speed of light (3.00x108 m/s). However, unless one plans on becoming a Muggle astronaut or cosmologist, only a very basic understanding of these phenomena are necessary for one’s success in this course.

Because time is a physical quantity, it therefore must be capable of measurement. Throughout the ages, humans have come up with many different systems for measuring time. The basic unit for time is the second. A second is one sixtieth of a minute. The length of a second originally was decided based on the Earth’s rotation around the Sun, until it was found that the second was actually lengthening. Since 1967, the length of one second has been determined by the decay rate of the caesium-133 isotope, using an atomic clock (a Muggle clock that uses the radiation emitted at regular intervals by an element to measure time). Units of time smaller than a second, such as the millisecond (10-3 seconds), microsecond (10-6 seconds), and nanosecond (10-9 seconds), also are measurable using atomic clocks, with each being 10-3 times smaller than the interval before it, though these units of time are extremely small, and do not come into play very frequently throughout one’s life.

7a. Units of Time

Getting into the units of time larger than a second, we see the minute, hour, day, week, month, and year. These intervals are especially important to understand, both for success in astronomy, as your ability to track the stars and planets depends upon it, but also simply to get by in your daily life.

Minute: One minute is the equivalent of sixty seconds. There are 1440 minutes in a day. The minute also can be used as a form of measurement in astronomy, to measure the angles of ascension of planets and stars in the sky.

Hour: An hour is equal to sixty minutes, or 3600 seconds. There are 24 hours in a day.

Day: Thus far, it appears as though the lengths of different units of time have been chosen arbitrarily, apart from the way in which they relate to one another. It is not so with the day. The length of one day corresponds to the amount of time it takes for the Earth to make one complete rotation on its axis, which is approximately 24 hours (it is actually about 23 hours, 56 minutes long on average, and fluctuates by a few microseconds each day). Exactly how this is measured will be discussed further in Chapter 11: Astronomical Measurements.

Week: A week generally is defined as a series of seven days. The seven-day week originated in Babylonia, in the 6th century BCE. The importance of seven days originally came from various Muggle religions, such as Judaism, which prescribes six days of work, and day one of rest. However, in astronomy, seven days represents about a quarter of lunation (the time it takes for one complete lunar phase cycle). The problem with this theory as to the origin of the seven-day week is that seven days actually only represents 23.7% of lunation, so the cycle gets out of sync with the days very quickly. Mathematically, a six-day week would make much more sense. In fact, although the seven-day week is by far the most popular, groups of people throughout history have used weeks ranging from three to ten days.

Month: The length of one month relates directly to the phases of the Moon. There are different ways of measuring a lunar month. A sidereal lunar month is the length of lunation in relation to fixed stars visible in the sky. It equals approximately 27.3 days. A synodic lunar month is the period between two New Moons, and is based on the position of the Moon with respect to the Earth and the Sun. This makes the length of a synodic month slightly longer than that of a sidereal month, at approximately 29.5 days. There are also solar months, which are equal to one-twelfth of a year, or about 30.3 days. Solar months also are called tropical months, and are used in most modern systems for keeping track of time.

Year: One year is equal to the time it takes for the Earth to make one complete revolution around the Sun. There are 365.25 days in a year. Because of the axial tilt of the Earth, throughout a year, the Earth experiences four distinct seasons, as well as the lengthening and shortening of the days, two solstices, and two equinoxes. This is called the tropical year. There is also the sidereal year, which can be measured as the rotation of the Earth around the Sun in relation to a star. The sidereal year is about 20 minutes longer than a tropical year. A lunar year is the length of time it takes for the Moon to go through twelve phase changes, and is equal to 354.37 days.

7b. Tools for measuring time

There are a number of different timekeeping devices, such as the hourglass, clock, and sundial, which you most likely are familiar with already. These are all good for counting seconds, minutes, and hours, but if one wants to keep track of a larger length of time - for example, to see how the constellations change throughout the year - a calendar becomes necessary. So many different calendars have been used throughout the ages that it is both impossible and unnecessary to learn them all. The main ones will be highlighted in the following paragraphs.

Solar calendar: A solar calendar marks the position of the Earth as it rotates around the Sun. The duration of a solar calendar is a tropical year. In a solar calendar, a date is assigned to each solar day, which is the time from sunrise to sunset. Solar calendars have 365 days. The two most commonly-used calendars in the world today, the Julian and Gregorian calendars, are both solar calendars. They both have twelve months, with each month having either 30 or 31 days, except for February, which has either 28 or 29 days.

Lunar calendar: A lunar calendar is based on the lunar year, which means it uses the transition of the Moon through its phases to measure time. A common lunar calendar is the Islamic calendar, which has twelve months, and a year that is 354 or 355 days long. Because it follows only the Moon, the Islamic calendar does not follow the seasons.

Lunisolar calendar: The lunisolar calendar takes into account both the solar and lunar calendars. This means it includes both the phases of the Moon, and the seasons. This calendar has twelve months, but every few years a thirteenth month is added. Whether or not an additional month is necessary can be determined based on the state of the crops or plants, and the movements of the stars. It also can be determined mathematically.

Gregorian calendar: A solar calendar with a seven-day week and twelve months, from January to December, used within the wizarding world. This is the same calendar used by most Muggles worldwide. However, there is a significant difference between the wizarding and Muggle systems for keeping track of hours, minutes, and seconds. Muggles use a system called coordinated universal time (UTC). UTC divides the globe into twenty-four main time zones, with a few intermediate time zones that only differ by a half hour or so. Each main time zone is one hour behind the time zone to its east. A number of places around the world also follow daylight savings time, which means in the spring season, they set their clocks forward one hour, and in the fall season, they turn the clocks back one hour. The reason for this is to stay better in tune with the solar day, so that one is awake when the Sun is up, and sleeps when it has set. However, the practice of daylight savings time is seen by many as unnecessary, and there are many places that no longer observe it. For wizards, rather than following the Muggle time zones or using daylight savings time, we simply determine the time based on the Sun’s position in the sky. This means wizarding time is always slightly different from Muggle time, as the position of the Sun is different depending upon the observer’s position on Earth. This still allows wizards to follow the solar calendar, and it lets us be more in sync with the seasons.

7c. Leap year? Leap day? Leap second?

Every once in a whilst, an extra unit of time must be added to the calendar so the calendar year of 365 days stays in sync with the seasonal or astronomical year of 365.25 days. This means every four years, an extra day must be added to the month of February, which only has 28 days, giving it 29 (at least in the Gregorian and Julian calendars). This is called a leap day, and the year to which it is added is called a leap year. A leap second is a one-second adjustment (or ‘leap’) that is added to the UTC time scale on either June 30 or December 31, to keep the UTC time scale close to mean solar time. The length of the solar day is slowly increasing, and when the official length of the second was set in 1967, it was already a bit short. As a result, the ‘solar second’ was adopted. Whether or not the addition of a leap second is necessary is determined using an atomic clock. Theoretically, a negative leap second could be added (in other words, a leap second could be subtracted) from the time scale, but this has never been necessary. In fact, over long time periods, leap seconds must be added more and more. Leap years and days are observed by those in the wizarding community so as to not become completely out of tune with the Muggle year. However, wizards have never utilised the leap second, finding the practice unnecessarily troublesome. This has not yet become a problem, however, as most people’s clocks are more than one second slow or fast anyway.

The Sun

The Sun, the centre star of our solar system, is roughly 109 times the size of our Earth (with a diametre of 1,392,000 km), with 333,000 times our mass. It is made of very hot plasma interwoven with magnetic fields. The stellar classification of the Sun is that of a yellow dwarf, although the Sun is technically white. The yellow Colour we see is caused by the scattering of its blue light by our atmosphere. Its temperature is about 5778 Kelvin (that’s about 5505 degrees Celsius or 9941 degrees Fahrenheit), and gains most of its energy from nuclear fusion that takes place in its core. Fusion is the combining of smaller nuclei to form a larger nucleus. On the Sun, fusion combines hydrogen atoms into helium atoms. At its centre, the Sun can fuse 620 million metric tons of hydrogen in one second. The fusion process gives off heat, light, and radiation. About 98.31% of the Sun is made up of hydrogen and helium, with the rest being made up of heavier elements such as oxygen, carbon, iron, and neon. The mass of the 1.69% composition of heavier elements alone is still 5,628 times the mass of our Earth.

7d. The Ecliptic

The ecliptic refers to the path the Sun follows over the course of one year, as traced on the celestial sphere. The celestial sphere is an imaginary sphere based on the ancient belief that the Earth was at the centre of the Universe. It can be thought of as an imaginary bubble around the Earth, imitating our same axis and tilt, upon which we view the stars and planets. This imaginary sphere is an often-used tool in spherical or positional astronomy. The ecliptic path of the Sun does not refer to the hour-by-hour movements that we see on Earth over the course of a solar day, but to the changes in the Sun’s position when viewed at the same time each day over a period of days. The term ‘ecliptic’ refers to the point when our Moon meets the Sun on its journey, creating an eclipse. (See the chapter on The Moon for more on eclipses.)

The Sun does not actually move whilst it is on the ecliptic path; rather, it is the Earth’s orbit around the Sun that causes the ecliptic and the Sun’s changing positions in our sky throughout the year. As we look at the position of the Sun from our Earth through the imaginary celestial sphere, it appears as if we are stationary when the Sun is moving, although the exact opposite is true. The Earth is really the one moving around the Sun, but our position in orbit throughout the year is marked by the illusion of the Sun’s ecliptic journey through thirteen zodiac constellations, as the Sun’s path is viewed from our vantage point here on Earth.

The celestial equator, in terms of celestial latitude and longitude, mirrors our own equator here on Earth, as if we were projecting our own equator into space. However, because of the Earth’s axial tilt, the celestial equator is inclined by about 23.4 degrees in relation to the ecliptic path. Celestial latitudes and longitudes are measured similarly to those used here on Earth. The longitudinal angle is measured going eastward from 0 to 360 degrees, whilst the latitudinal angle is measured positive northward of the equator, and negative southward. The Sun is said to be at position 0 longitude whilst in the constellation of Aries, and appears to move a little under 1 degree per day to the east on the celestial sphere, making the full 360-degree journey across the ecliptic over a course of 365.25 solar days. The Sun also appears to ‘bob’ over and under the celestial equator, in terms of celestial latitude, throughout the course of the year due to our own axial tilt. The ecliptic path meets the celestial equator, and the Sun is in position 0 on the latitudinal scale, during both the vernal and autumnal equinoxes.

7e. The Zodiacs

The term ‘zodiac’ comes from the Latin zodiacus, which was derived from the Greek words zodiakos kyrklos, meaning circle of little animals. This division of the ecliptic path into zodiac constellations dates as far back as the first millennium BC, from the Babylonian astronomy of Mesopotamia. Many of the zodiac constellations were represented with animals at that time, just as they are now.

There are thirteen zodiac constellations on the Sun’s ecliptic path. The Sun never actually ‘occupies’ a constellation as a physical body – it just appears that way to us, as we view the Sun and the constellations from Earth. The Sun is declared to be ‘in’ a particular sign (for instance, when the ‘Sun is in Aries’) when its position, as seen in the celestial sphere, is inside one of these constellations of stars. As we move around the Sun during the course of the year, we see the Sun in the position of each of the thirteen zodiac constellations for a particular period of time. These periods of time can be as long as a month-and-a-half, to as short as about a week.

The dates in which the Sun is in each constellation on its ecliptic path are shown in the following chart:

Sagittarius December 18 - January 18

Capricornus January 19 - February 15

Aquarius February 16 - March 11

Pisces March 12 - April 18

Aries April 19 - May 13

Taurus May 14 - June 19

Gemini June 20 - July 20

Cancer July 21 - August 9

Leo August 10 - September 15

Virgo September 16 - October 30

Libra October 31 - November 22

Scorpius November 23 - November 29

Ophiuchus November 30 - December 17

Twelve of the thirteen constellations – all but Ophiuchus - traditionally have been used in popular astrology. Only about 1% of modern astrologists acknowledge the thirteenth zodiac constellation, Ophiuchus, as a ‘zodiac sign.’ This statistic only includes human astrologists, however, as the Centaurs, notorious for their secrecy, have refused to provide their input on the topic.

Astrology and divination are completely different sciences from astronomy, although they do intertwine, as the thirteenth zodiac in the astronomical sense is not omitted from inclusion on the Sun’s ecliptic path. The astrological zodiac signs, or the ‘Tropical Zodiacs,’ vary a great deal from the astronomical positions of the Sun and the zodiac constellations. The zodiacal signs in reference to astrology take a more abstract approach to the physical constellations, by placing them evenly at 30 degrees apart, with three zodiacs per season. However, the distance of the constellations from each other in an astronomical sense is not so unified. Because of the varying sizes and positions of the constellations, the Sun spends very uneven amounts of time ‘occupying’ each zodiac constellation, as shown in the chart above.

7f. The 13th Constellation: Ophiuchus

The first mention of Ophiuchus, coming from the Greek term meaning ‘serpent-bearer,’ was in the 4th century BC by the Greek poet Aratus, in his hexametre poem Phaenomenaor ‘Appearances.’ In the first half of the poem, Aratus describes the positions of the constellations and other celestial phenomena. This portion of the poem is thought to be based on two prose works written about a century earlier by the wizard astronomer and mathematician Eudoxus of Cnidus. In Phaenomena, Aratus writes of Ophiuchus:

‘To the Phantom's back the Crown is near, but by his head mark near at hand the head of Ophiuchus, and then from it you can trace the starlit Ophiuchus himself: so brightly set beneath his head appear his gleaming shoulders. They would be clear to mark even at the midmonth Moon, but his hands are not at all so bright; for faint runs the gleam of stars along on this side and on that. Yet they too can be seen, for they are not feeble. Both firmly clutch the Serpent, which encircles the waist of Ophiuchus, but he, steadfast with both his feet well set, tramples a huge monster, even the Scorpion, standing upright on his eye and breast. Now the Serpent is wreathed about his two hands – a little above his right hand, but in many folds high above his left.’

According to Muggle mythology, the constellation represents the healer Asclepius, who found the secret to immortality after watching one serpent bring another healing herbs. According to one myth, after Asclepius brought people back from the dead, Zeus killed him with a thunderbolt to prevent the entire human race from becoming immortal, but in honour of his noble work, Zeus placed the image of Asclepius with the serpent forever in the stars, as the god of medicine and healing. However, we now know that the wizard and early alchemist Asclepius was not killed by Zeus, but instead chose to stop taking the herbs, as he wished to come to terms with his time. He decided to keep his work a heavily guarded secret, only passing it on to his most trusted descendants. The last known descendant of Asclepius was none other than Nicolas Flamel, who no doubt used Asclepius’s secret herbal formula in his making of the Philosopher’s Stone. The constellation Ophiuchus was created along the ecliptic plane, using an ancient and mysterious form of magic, as an immortal homage to the true creator of the Philosopher’s Stone.

Ophiuchus is also known as Serpantarius, as Asclepius was a known speaker of Parseltongue, and was believed to have worked extensively with snakes in his research. Asclepius is shown in the constellation holding a giant serpent.