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Definitions and Terminology
Published in Frank Vignola, Joseph Michalsky, Thomas Stoffel, Solar and Infrared Radiation Measurements, 2019
Frank Vignola, Joseph Michalsky, Thomas Stoffel
From an Earth-centered point of view, the Sun moves among the other stars in the heavens on the celestial sphere (see Figure 2.3). The observer on Earth is stationary, and the objects in the heavens look as if they are on the celestial sphere rotating at a rate of just under 360 degrees per day (360°/d) or, equivalently, 15 degrees per hour (15°/h). The plane running through the Earth’s equator is designated the celestial equator (or equatorial plane), and the (celestial) polar axis, identical to Earth’s polar axis, is the axis of rotation of the celestial sphere around the Earth. This plane and axis extend to infinity. Similar to the fact that any location on Earth can be specified by the latitude and longitude of the location, the position of any star on the celestial sphere—the Sun in this discussion—can be determined by specifying two parameters: the right ascension and the declination. The difference is that the locations on Earth are fixed while the location of the stars change over the day and over the year as the Earth spins on its axis and takes its journey around the Sun. For the observer on Earth, the stars or Sun look like they are moving in a plane that is parallel to the equatorial plane at a rate of about 15°/h.
Introductory Material
Published in Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr, Fundamentals of Radio Astronomy, 2019
Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr
The declination coordinate is measured in degrees (°), minutes of arc (’) and seconds of arc (”), where there are 60 minutes of arc in a degree and 60 seconds of arc in an arcminute. The projection of the equator (latitude 0°) onto the sky defines the celestial equatorCelestial equator and following the convention for latitude, the celestial equator is defined to be the 0° declination line. The projections of the Earth's north and south poles on to the sky define the north celestial poleNorth celestial pole (at declination +90°) and the south celestial poleSouth celestial pole (at declination −90°), respectively. Following the definition of latitude, the value of the declination is the angle between the celestial equator and the line of declination as viewed from the center of the Earth. Angles north of the celestial equator have positive values of declination, while angles south are negative.Declination (Dec)
Modelling available solar radiation
Published in David Thorpe, Solar Energy Pocket Reference, 2018
Declination (δ) is the angle of an object in the sky with reference to the perpendicular to the celestial equator. The celestial equator is a projection from a point at the centre of the earth of its equator onto the celestial sphere. By convention, angles are positive to the north, negative to the south. The declination angle of the sun in the sky varies throughout the year due to the tilt of the earth on its axis of rotation as it circumnavigates the sun and gives us the seasons. The axial tilt is 23.45° and the declination angle varies plus or minus up to this full amount. At the equinoxes the value is 0°, at the 22 December solstice it is −23.45° and at the 22 June solstice it is 23.45°. It is given by the equation: δ=23.45°⋅sin[360/365⋅(284+N)]
Guillaume des Moustiers’ treatise on the armillary instrument (1264) and the practice of astronomical observation in medieval Europe
Published in Annals of Science, 2021
Guillaume’s main text closes by mentioning some computational uses of the armillary instrument that appear neither in Ptolemy’s original account nor in the Almagesti minor: finding the solar declination, the co-culminating degree (or mediation) of a star, ‘and other properties that arise for them from the motions of separate [bodies]’.49 The glosses add some substance to these extremely concise remarks by explaining how to determine stellar mediations and solar declinations. To find a star’s co-culminating degree, one simply moves the sighting vanes of the innermost ring (1 in Figure 1) from the star itself to the meridional ring (6) to then look for the degree on the ecliptic ring (3) that is being intersected by the meridional ring at this instance. In a subsequent step, the vanes can also be used to gauge the altitude of the star at its culmination.50 Once the ecliptic degree of the Sun has been aligned with the meridional ring, it will be possible to use the same ring to measure the solar declination, provided one knows which point on its surface corresponds to its intersection with the celestial equator.51
Construction of sundials via vectors
Published in International Journal of Mathematical Education in Science and Technology, 2019
The angle φ in Figure 1 is the latitude and d the declination of the Sun (angular distance from the celestial equator). The angle d varies with the inclination of the axis of the Earth, between approximately to during one year. Since the distance from the Sun to the Earth is large, light rays from the Sun striking the Earth can be assumed to be parallel (illustrated by the lines and ). The style in Figure 1 is oriented in the north–south direction pointing towards north with an angle φ to the horizontal plane (this plane is tangent to the sphere at the point B). The reader is encouraged to show that the angle BDC equals φ implying (use alternate angles) that the style is parallel to the axis of the Earth, and that the height of the Sun, ABE, is . The reader is also advised to think through the construction when the latitude satisfies . We work on the northern hemisphere and assume hereafter that the latitude is greater than .
Utilizing dynamic shading system to achieve daylight performance according to LEED standards V.4: case study, university classrooms in Egypt
Published in HBRC Journal, 2021
Illumination metrics discuss the adaptive and spatial elements of daylight. In 2012, the Lighting Engineering Association (IES) published an accepted method for daylight efficiency measures (LM-83 12 (2012) and then released LEED v4 in 2014, and the daytime balance aims to maximize daylight in zones. There are three options for measuring daylight performance by LEED: Option 1, Simulation: SDA (300 lux, 50% hour) and ASE (1000 lux, 250 hours). Option 2, Simulation: Illuminance Levels (300–3000 lux, Equinox (The time or date at which the sun crosses the celestial equator, when day and night are of equal length (about September 22 and March 20). at 9 am and 3 pm. Option 3, Measurement: Illuminance Levels (300–3000 lux, at 9 am and 3 pm) [8, LEED, U 2014 [9]].