![]() ![]() The history of the Western calendar The Roman calendar ![]() A lunar calendar consisting of 354 days (12 lunations) would keep in step with the moon-with some days added in from time to time-but would very soon get out of step with the year and, therefore, the seasons.Īll calendars were-and still are-plagued by the lack of synchrony between the moon’s cycle and the length of the year, and by the fact that neither the length of the solar year nor the length of the lunar month is a whole number. This was not an easy task, as there are about 12.368 lunations in a solar year. Lunisolar calendars attempted to keep in sync with both the moon and the solar year. Year lengths were also determined by counting the days between two equinoxes. This was refined by interpolation between readings on successive days around the summer solstice, and by the construction of ever-larger gnomons, which provided increasingly accurate estimates of the exact time of the solstice. ![]() Image source: Bernat / Wikimedia Commons.Īs the shadow cast by a vertical gnomon is shortest at noon on the day of the summer solstice, a count of the days between two summer solstices would give an estimate of the length of the year. A sundial made in the Joseon Dynasty era, displayed in South Korea. Sundials were first developed by the ancient Egyptians. The shadow cast by the gnomon tracks across the sundial as the sun moves across the sky, and is used to tell the time of day. One of the most common ways of measuring the length of a year in ancient times involved the use of a gnomon-a structure that casts a shadow, like the vertical stick or triangle in the centre of a sundial. Solar calendars have similar issues to lunar calendars.Įarly astronomers used solstices (when the sun is at its furthest from the equator) and equinoxes (when the sun crosses the plane of Earth’s equator) as starting and finishing points. Other calendars measure time in terms of how long it takes Earth to complete one circle of the sun-this is a solar calendar. This is usually done by periodically adding days (intercalations) or subtracting days (extracalations). So what happens is that these calendars must be ‘adjusted’ from time to time. ![]() Trying to solve the problem by alternating the length of the month between 29 and 30 days, giving an average month of 29.5 days, still results in a calendar that gets out of step pretty quickly, since the actual length of a lunation is a bit more than 29.5. The first month would be out of sync by around half a day and the next month by a full day. If ‘29’ were the number used to mark the lunar month, the calendar would very quickly get out of sync with the actual phases of the moon. Lunar calendars are problematic, due partly to the fact that the average lunation is not a whole number. However, this demarcation can seem somewhat arbitrary, particularly for cultures where people’s lifestyles are more in tune with nature, where a more intuitive marker for the beginning of a new day would be the sunrise. In the Western world, we are accustomed to starting a ‘new’ day at midnight-which is approximately in the middle of the night (presuming the sun sets at around 6 pm and rises again at around 6 am). A sidereal day works out to be around 4 minutes shorter than a solar day. The second way we can measure a day is when distant stars appear back in the same position after moving through the sky. This gives us a mean solar day of 24 hours. The speed used is the average speed of the apparent motion of the sun through the sky (its ecliptic motion-the path it traces out in the sky as Earth orbits it). To get around this, we use a pretend sun that is moving at a constant speed around Earth’s equator. The fact that Earth is also orbiting the sun, and does not move at a constant speed throughout its orbit, means that the time taken for the sun to be back in its same position is not consistent for each rotation. The first is by noting the length of time taken for the sun to appear in the same position in the sky after one rotation, say, from noon to noon. There are two ways to mark the completion of a full rotation. As it rotates, the portion of the planet facing the sun changes, hence the difference between night and day, and the fact that the transition between light and dark takes place at different times around the globe. In short, in the Western calendar, a day is the amount of time it takes Earth to complete one full rotation on its axis. ![]()
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