FAQs

The equinox is defined as the moment at which the plane of the Earth equator passes through the centre of the Sun. In other words, it is the moment at which the center of the visible Sun is directly above the equator. The northern and southern hemispheres of the Earth are then illuminated in the same way. This moment occurs two times each year: around 20 March (spring equinox) and 23 September (autumnal equinox). However, during the equinox, the day and the night are not of equal length. Actually, the length of the day is slightly longer than the length of the night. Why is this the case? This is mainly for two reasons:
  • The Sun is not perceived from the Earth as a point but as a luminous disc. Sunrise (sunset) is defined as the time when the upper edge of the solar disc, not the centre of the Sun, appears (disappears) on the horizon. Therefore, since the upper edge of the Sun appears before its centre, and disappears after its centre, we gain a few minutes of sunlight during the equinox.
  • Sunlight is deflected by the atmosphere. This is due to atmospheric refraction, a phenomena that “flattens” the Sun near the horizon. As a result, the upper edge of the Sun may be visible when it is actually below the horizon.
Illustration of the Sun refraction
Illustration of the Sun refraction phenomena.
These two effects combined therefore explain why the length of the day is longer than 12 hours on the day of the equinox. The day during which the length of the day is closest to 12 hours is before the spring equinox and after the autumnal equinox. In Belgium, this day is the 17th or 18th of March and the 25th or 26th of September.
If you have questions about weather or climate, you can consult the web pages of the Royal Meteorological Institute (KMI-IRM).
Since 2018, we no longer give information on religious calendars. We now only give information on the Belgian civil calendar, which is based on the Gregorian calendar.
If you are looking for concrete information about sea tides on the Belgian coast and on the inland rivers, please contact the Ministry of the Flemish Community, Afdeling Kust - Vlaamse Hydrografie, Vrijhavenstraat 3, 8400 Oostende (059/554211) and/or Afdeling Maritieme Schelde, Tavernierkaai 3, 2000 Antwerpen (03/2220803). Some websites also provide information about this, e.g. http://www.vlaamsehydrografie.be or http://www.lin.vlaanderen.be/awz that gives all water levels. At the Royal Observatory the earth tides are measured and studied using gravimeters. The small differences between the astronomical calculations (position sun, earth, moon) and the gravimetric measurements are investigated in detail to detect any other external influences. More information: https://www.astro.oma.be/en/scientific-research/seismology/.
The maximum activity of the Perseids meteor shower usually occurs around August 11 or 12. Meteors (or shooting stars) can then be observed throughout the night, as well as during the previous and subsequent nights, preferably in the second part of the night, when the Moon is down and the Perseus constellation is high enough in the sky. Meteors seem to come from the constellation of Perseus - hence their name - but this is only a perspective effect. Meteors are luminous phenomena that result from the passage of a solid body from space through the Earth's atmosphere. These are actually dust corpuscles associated with the Swift-Tuttle comet's passage through its orbit. Every year, around August 11 or 12, the Earth passes through this orbit and dust particles left by this comet enter the Earth's atmosphere. These dusts are usually no larger than grains of sand entering our own atmosphere at high speed (up to 60 km/s). They leave bright trails in their path before disintegrating completely before they can reach the ground of our planet. These luminous trails can be observed everywhere in the sky but seem to come preferentially from the constellation of Perseus, which is then above the northeast horizon. In areas with very high light pollution, only the brightest meteors can be observed. In regions with relatively dark skies, up to one meteor per minute can be counted on average during periods of maximum swarm activity. The period of the normal maximum is actually very long in time (over several weeks) because the dust particles have been dispersed for thousands of years in their orbit. This is why it is possible to observe many meteors over a fairly long period stretching before and after the period of the theoretical maximum.
A meteor or shooting star is caused by a particle that enters the Earth's atmosphere from space. The particle is sometimes called meteoroid. Through the interaction with the atmosphere, light and heat are released and a so-called ionization trail is formed (from 100km of altitude). This is visible as the "tail" of the shooting star. The particle almost always evaporates or pulverises in the high atmosphere (above 20-50 km altitude). Most particles that give rise to meteors are not larger than a few milimeters. A clear meteor, also known as a fireball or a racing car, is a particle that is larger (several centimmeter or more). Often the fireball comes to its end at an altitude of 10 to 20 km. Sometimes this is accompanied by an explosion (without sound) and one also sees different colors. The larger the particle, the brighter the fireball, but also the speed (typically between 10 and 70 km/s) and the composition play a role. Sometimes sounds (pops) are heard during the passage in the atmosphere. Only very rarely does a residue end up on the Earth's surface. That is called a meteorite. There are many species, but they are very rare.
The length of the year in the Western calendar has its origin in the Roman calendar. Since 46 BC, Julius Caesar wanted to establish 365-day years with a leap year every 4 years. However, it will take some time before the Julian calendar is properly applied. The scope of this calendar refers to the length of the tropical year. This is the time it takes for the Sun to return to the same position in the sky. During this time, the Sun moves from its northernmost position to its southernmost position relative to the equator and vice versa. This results in the frequency of the seasons that the calendar wants to reflect. A tropical year lasts 365.242190 days or 365 days 5 hours 48 minutes 45.2 seconds, so it is not an integer number of days. A good approximation was made by introducing a year of 366 days every 4 years as already provided for in the Julian calendar. This gives a year with an average duration of 365.25 days. Over time, a small difference (0.0078 days per year) began to be a problem. Thus, because of the imperfection of the Julian calendar, spring in the 16th century was moved to March 11. In order to put everything in order, Pope Gregory XIII decided in 1582 that October 4 should be followed by October 15. In addition, at the request of the astronomers of the time, he implemented the following new rule: a year divisible by 100 will not be leapfrogging, unless it is a multiple of 400. the first rule alone produces an average year of 365.24 days every 100 years. The second rule is to get very close to the tropical year, i.e. 365.2425 days every 400 years. It is only after 3000 years that the inaccuracy deviates by one day. However, this timetable was not immediately adopted by all countries. Russia waited until 1917, which is why the October revolution is now commemorated in November. Sweden became so confused in the 18th century that it was necessary to give 30 days in February in 1712. Greece adopted the Gregorian calendar in 1923. Several people noted that some sources contradict each other. Thus, the southern regions of present-day Belgium moved to the Gregorian calendar from 1582 to 1583. In some places (Flanders and Hainaut) Christmas Day was not celebrated in 1582, since December 21, 1582 was immediately followed by January 1, 1583. Even if Brabant and Zeeland had made the transition earlier (14-25 December), it was not without difficulties. The Liege échevinage introduced the Gregorian reform in February 1583. You can find more information about calendars on, for example, The Calender FAQ.
A full century is 100 years old. 100 years must therefore pass before a new century can begin. Since our era begins with the year 1 AD, the 1st century ended with the end of the year 100. Thus, the year 1900 had to end first before the 19th century could be finished. The 20th century also lasted 100 years and ended with the beginning of 2001. The reasoning is the same for a millennium. A millennium lasts 1000 years. The second millennium began on 1 January 1001 and ended on 31 December 2000. Each era begins with year 1. There is no such thing as year 0.

A brief historical overview

Among the Romans, who laid the foundations of our calendar, the era began with the foundation of Rome, fixed in 753 BC (Ab Urbe condita or AUC). However, there were other starting dates. Since the 3rd century AD, the years have been calculated from the ascent of the Emperor Diocletian. Since he persecuted Christians, this time was called the era of the martyrs. In the 6th century, the monk Dionysius Exiguus (Dionysius the Little) began to count from the year that, according to him, was the year of Christ's birth. Thus, 248 Anno Diocletiani was in fact the year 532 AD. According to Dionysius, Jesus Christ was born on December 25 of the year before year 1, he did not know the number zero, but there is no era in which a year 0 would be placed. In the 8th century, the English Father Bede the Venerable also began to count the years before Christ. The year 1 BC was therefore just before the year 1 AD. The year 1 BC was therefore the last year of the 1st century BC. It should be noted that this Christian way of counting years was used by the Church in the Middle Ages and much later in the civil world. There were many other ways to count. There were controversies about the date of the beginning and in particular about the year of Christ's birth and Herod's death in 4 BC, which should be brought forward by a few years, to around 6 or 7 BC. You can find more information about calendars on The Calender FAQ web page.
The (Catholic) Easter date was fixed by the Council of Nicea in 325 AD. The rule is as follows: Easter is the first Sunday after the first full moon of spring. Taking into account modern astronomical calculations, in 2019,
  • spring begins on March 20 at 22:58 (universal time);
  • the first full moon that follows takes place on March 21 at 2:43 (universal time).
Therefore, Easter would fall on the first Sunday following, i.e. on March 24, 2019. However, the Easter date in 2019 is April 21. The reason why the two dates do not coincide is that the authorities of the Catholic Church do not want to depend on astronomers. For this reason, they have introduced the following simplifications into their calculations: 1) Early spring (in the northern hemisphere): In astronomy, it coincides with the vernal equinox, the moment when the terrestrial equatorial plane crosses the centre of the Sun and the Earth's North Pole emerges from its winter night. The equinox date fluctuates in the calendar, in part because the Earth's period of revolution around the Sun is 365.2422 days instead of 365-366 calendar days (other factors influence the calculation of the spring date, but their influence is too small to be mentioned here). As a result, early spring is between March 19 and 21. The church has decided to set the date from spring to March 21. 2) The dates of the full moon: It is not easy to determine the exact orbit of the Moon and the corresponding lunar phases. In the 5th century BC, the Greek Meton of Athens found an elegant solution to simplify this problem. After a number of observations he concluded that the Sun and Moon are in the same relative position with respect to the Earth after 235 lunar months, or 19 years. This cycle has become the basis for the calculation of the full moon in the Catholic Church. In 1582, this rule was adapted to the new Gregorian calendar, our current calendar. The rule for calculating the Easter date therefore remains valid, but we must also take into account the two simplifications mentioned above. In other words,
  • spring begins on March 21;
  • the first full moon after March 21, 2019 (according to Meton calculations) takes place on April 20, 2019.
Therefore, Easter falls on the Sunday following this full moon, i.e. on April 21, 2019. In addition to 2019, there are other years when the "ecclesiastical" Easter date does not correspond to the "astronomical" Easter date. In the thousand years following 1582, the year of the Gregorian calendar reform, there were 84 such occurrences. The previous date was in 1981, the next will be in 2038. The German mathematician Carl Friedrich Gauss published in 1800 a mathematical algorithm to calculate the Easter date for a given year. He uses different terms of the comput such as the golden number or the epacte. This algorithm is still used today, sometimes in a simplified version. Links: Easter dates from 1583 to 3000 Algorithm for the computation of the Easter date