EDUCATIONAL ACTIVITY 2. Calculating the Latitude of the Observation Site from Images of the Midnight Sun
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EDUCATIONAL ACTIVITY 2. Calculating the Latitude of the observation site from images of the Midnight Sun. Mr. Miguel Ángel Pío Jiménez. Astronomer Instituto de Astrofísica de Canarias, Tenerife. Dr. Miquel Serra-Ricart. Astronomer Instituto de Astrofísica de Canarias, Tenerife. Mr. Juan Carlos Casado. Astrophotographer tierrayestrellas.com, Barcelona. Dr. Lorraine Hanlon. Astronomer University College Dublin, Irland. Dr. Luciano Nicastro. Astronomer Istituto Nazionale di Astrofisica, IASF Bologna. 1 – Objectives of the activity. In this activity we will learn to calculate the latitude of an observer from digital images and calculating in them the height the Sun on the horizon. The goals of this activity are to: - Explain the phenomenon of the midnight Sun. - Apply a methodology for the calculation of a physical parameter (Geographycal Latitude) from an observable (digital images). - Support the teaching and learning of mathematics and physics by applying knowledge of mathematics (algebra and trigonometry) and basic physics (kinematics) to solve a practical problem. - Understand and apply basic techniques of image analysis (angular scale, distance measurement, etc.). - Work cooperatively as a team, valuing individual contributions and expressing democratic attitudes. 2 – Instrumentation. The activity will use digital images obtained from northern Europe during the transit of Venus on June 6, 2012 (see sky-live.tv). 3 – Phenomenon. 3.1. Midnight Sun. The Midnight Sun is a natural phenomenon that occurs in its fullness only north of the Arctic Circle and south of the Antarctic Circle. Since in the southern hemisphere there are no permanent settlements sufficiently close to the pole (except in the Antarctic bases which are inhabited by a few scientists and military personnel), inhabited regions that can enjoy with this phenomenon are all in the Northern Hemisphere: Alaska, Canada, Greenland, Norway, Sweden, Finland, Russia and northern end of Iceland. Due to the inclination of the Earth's axis of rotation to the ecliptic of about 23 degrees and 27 minutes, at these high latitudes the Sun does not set during the summer. Midnight Sun 1 Figure 1: Render created to simulate a sequence of photos of the midnight Sun taken over 24 hours. The Sun is at its maximum height at noon, and its lowest height at midnight. Credits: Anda Bereczky. The midnight Sun remains visible above the horizon at these latitudes (see Figure 1). The length of time for which the midnight Sun remains visible depends on the latitude. For example, at the Artic circle (latitude 60 degrees) it is visible for only 20 hours around the summer solstice (between June 22nd and 24th), while at the geographic poles it is visible for 6 months. At the poles over the course of a year, sunrise and sunset are observed only once. During the six months of day at the pole, the Sun moves continuously near the horizon, reaching its maximum height in the sky at the summer solstice. Due to refraction (bending of sunlight through our atmosphere), the midnight Sun can be observed at latitudes slightly below the polar circle, but at most one degree below (depending on local conditions). For example, the midnight Sun can be seen in Iceland, although most of the country (the island of Grimsey is the notable exception) is located to the south of the Arctic Circle. In the most northerly of the British Isles (and other places at similar latitudes) the sky doesn’t get completely dark at this time of year. As mentioned, the duration of the midnight Sun varies, depending on latitude. For example, at the northernmost point of mainland Europe, the North Cape in Norway, the midnight Sun can be enjoyed from May 14th until July 29th. However, slightly further south, at the latitude of the Arctic Circle, in Rovaniemi, a place referred to as the reference of the circle, it is only observable from June 12th to July 1st. A quarter of Finland's territory lies north of the Arctic Circle and in the northernmost part of the country the Sun does not set for 73 days in summer. In the Svalbard Islands, Norway, the northernmost inhabited region of Europe, there is no sunset from around April 19th to August 23rd. At the extremes are the poles themselves, where the Sun remains visible for half of the year. After this time, the Sun remains above the horizon for several hours each day, and then sets. The nights therefore get progressively longer, until the opposite time of year is reached, when the Sun sets for months, leading to the long polar night. In order to prepare to observe and enjoy this wonderful phenomenon, the best time will be between about 23.00 and 01.00 hours from Norway. This is the time when we can see how the Sun descends towards the horizon, acquiring the typical orange colour of a conventional sunset, but then, rather than hiding, will reach a point where it will start to climb again, transforming sunset into sunrise. For the rest of the “day”, the Sun is never seen high overhead as we are used to seeing from other latitudes during summer. Therefore, during this period, and just above the Arctic Circle, the Sun is not as intense as would be expected at this time of year. Midnight Sun 2 3.2. The White Nights. Northern regions of Russia and other locations that are above 60 degrees latitude, but are south of the Arctic Circle (or north of the Antarctic Circle in the southern hemisphere) do not experience the midnight sun, but experience civil twilight at midnight. The Sun sets, but doesn’t go more than 6 degrees below the horizon (which defines civil twilight), so daytime activities, such as reading, are possible without artificial light, provided the sky is not cloudy. This natural phenomenon is popularly known by the name of the White Nights. Figure 2: Fireworks at the launch of the White Nights Festival in St. Petersburg, Russia. The sailboat with scarlet sails is the frigate Standart (built by Peter I of Russia). 3.3. The Polar Night. The polar night occurs during winter, when the Sun does not rise for a long period of time and the days are in darkness. In places like the Svalbard islands, the polar night extends from October 28th until February 14th. In the regions within the polar circles (latitudes greater than 66 degrees north or south), the length of time the Sun is below the horizon varies from 20 hours at the Arctic and Antarctic Circles, to 179 days at the north and south poles. However, all this time is not classified as polar night, as there may be some sunlight because of refraction. It is also worth noting that the Sun stays above the horizon for 186 days (compared to 179 below). The difference is just due to the fact that even if the Sun is only partially above the horizon, it is regarded as daytime. Midnight Sun 3 Figure 3: Night Pole at North of Europe. Longyerbyen, Svalbard, Norway. 4 – Methodology. 4.1. Sun apparent movement on the Celestial Sphere. The position of the Sun appears to change during the course of the year, so that the points where it rises and sets are constantly changing. In reality, of course, it is Earth’s position and orientation with respect to the Sun, which changes (Figure 4). However, here we adopt the convention in which the Sun is referred to as being in motion. On March 21st, at the vernal equinox, the Sun rises in the east and sets in the west. As the days pass, these points move northwards, first rapidly, then slowly, until June 21st, when at the summer solstice, the Sun reaches its maximum height over the horizon for northern latitudes. Sunrise and sunset are then at their most northerly positions. Midnight Sun 4 Figure 4: Representation of day-time and night-time as a function of the time of year and the orbit of the earth around the Sun. From June 21st, sunrise and sunset move gradually to their equinox positions, due east and west, on September 22nd / 23rd, the autumnal equinox. They continue to move southwards until December 22nd, the winter solstice. After that time, the sunrise and sunset positions begin their northward migration once again, returning to their spring equinox positions after one year. The Earth’s axis is tilted at an angle of 23°27' with respect to the line perpendicular to its orbital plane (Figure 5). The trajectory of the Sun across the sky and its declination therefore changes over the course of a year (Figure 6) where the declination of the Sun is the angle between the rays of the Sun and the plane of the Earth's equator. Figure 5: The Earth’s axis of rotation is tilted by 23o27' with respect to the North Pole. Credits: Wikimedia commons. Midnight Sun 5 Figure 6: The declination of the Sun over the course of a year (www.astro.virginia.edu/class/oconnell/astr130). Before the Sun crosses the equator on March 21st, its declination is negative (Figure 6), while at the vernal equinox on March 21st its declination is zero. Then, the maximum height of the Sun above the horizon is 90 degrees minus the latitude of the observer’s location, φ (i.e. 90 − φ). The length of the day is equal to the length of the night at the equinox. In the days following, the declination of the Sun is positive, and continues to increase until it reaches +23° 27', at the summer solstice when it is overhead at noon for the Tropic of Cancer. In the northern hemisphere this is the longest day and the shortest night of the year and the Sun reaches its maximum height above the horizon (= 90o − φ = 23° 27').