Welcome to Episode 85 Our Companion to Objects to Observe in the night sky for the Actual Astronomy Podcast. We are amateur astronomers that means we love looking up at the night sky and this podcast is for anyone who enjoys going out under the stars.
When I made up our eBook for objects to observe in the 2021 night sky, I jotted down some common or frequently occurring topics. Like…..
Sky Measurements – Degrees, Arc Minute and Arc Second
Minute and second denote time passage according to Babylonian Astronomers, the Babylonian degree of time was four minutes long, so the “minute” of time was four seconds long and the “second” 1/15 of a second. In practical terms, a degree is 1/360 of a turn while the arc minute is 1/60 of a degree and arc second 1/60 of the arc minute. The arcsecond is often used to describe small astronomical angles such as the angular diameters of planets while arc minutes and degrees describe larger sizes like constellations or comet tails. For beginners knowing planets are arc seconds in size and the moon is a ½ degree while the hand is 10-degrees and an index finger is a degree area ll you need to know.
Zenith – The zenith is an imaginary point directly “above” a particular location, on the imaginary celestial sphere. “Above” means in the vertical direction (plumb line) opposite to the gravity direction at that location (nadir). The zenith is the “highest” point on the celestial sphere.
Nadir – is the direction pointing directly below a particular location
Meridian – imaginary circle in a plane perpendicular to the planes of the celestial equator and horizon
Pole Star – A pole star or polar star is a star, preferably bright, nearly aligned with the axis of a rotating astronomical body. Polaris is the one we are most familiar with.
Ecliptic – the plane of Earth’s orbit around the Sun. From the perspective of an observer on Earth, the Sun’s movement around the celestial sphere over the course of a year traces out a path along the ecliptic against the background stars.
Celestial Equator – the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract projection of the terrestrial equator into outer space
Right Ascension & Declination – Right Ascension (RA) the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the). When paired with declination, these astronomical coordinates specify the location of a point on the celestial sphere in the equatorial coordinate system. Declination’s angle is measured north or south of the celestial equator, along the hour circle passing through the point in question
Magnitude – magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus. Astronomers use two different definitions of magnitude: apparent magnitude and absolute magnitude. The apparent magnitude (m) is the brightness of an object as it appears in the night sky from Earth.
Zodiac – an area of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year. The paths of the Moon and visible planets are also within the belt of the zodiac.
Opposition– two astronomical objects are said to be in opposition when they are on opposite sides of the celestial sphere, as observed from a given body (usually Earth). A planet (or asteroid or comet) is said to be “in opposition” or “at opposition” when it is in opposition to the Sun.
Conjunction – occurs when two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude, usually as observed from Earth. As seen from a planet that is superior, if an inferior planet is on the opposite side of the Sun, it is in superior conjunction with the Sun. An inferior conjunction occurs when the two planets lie in a line on the same side of the Sun. In an inferior conjunction, the superior planet is “in opposition” to the Sun as seen from the inferior planet.
Occultation – an event that occurs when one object is hidden by another object that passes between it and the observer.
Greatest Eastern and Western Elongation – a planet’s elongation is the angular separation between the Sun and the planet, with Earth as the reference point. The greatest elongation of a given inferior planet occurs when this planet’s position, in its orbital path around the Sun
Prograde & Retrograde Orbits – orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object (right figure). It may also describe other motions such as precession or nutation of an object’s rotational axis. Prograde or direct motion is more normal motion in the same direction as the primary rotates
Meteor Shower – Meteors are cosmit left overs from the creation of the solar system, typically the size of a grain of rice or smaller you might be lucky to see a few over the course of the evening from a dark site. Periodically throughout the year we have Meteor showers, Meteor Data Centre lists over 900 suspected showers. These originate from comets in orbit around the Sun and where the Earth intersects their orbits we have a shower of meteors surrounding certain dates. Because the meteors are travelling in parallel lines through space they appear to radiate from a common point in the sky and the showers are named after the constellations they appear to radiat from.
Comets – Small City sized objects (in relations to astronomical objects), in orbit around the sun, typically come from the Oort Cloud and gravity of a passing planet star, collision knocks them in towards the sun.. As they get in close to the sun they begin to offgas and a cloud of particles reflects the sun making them appear to brighten as they approach the inner solar system where the Earth is. Comets that are new and coming in from the far reaches and on elongated orbits are “long Period” while ones that orbit in under 200yrs are periodic.
Moon – Waxing and waning – When the lunar phases are progressing towards a Full Moon, this is referred to as waxing. When the lunar phases are progressing towards a New Moon, this is referred to as waning.
Lunar Terminator – Where the darkness of the moon transitions to the sunlite side. It moves throughout the evening , best spot craters, gives the most shadow definition.
Jupiter – Galilean Moon Shadow Transits – Io, Europa, Ganymede and Callisto continually orbit Jupiter, from our line of site we’ll see them pass in front of (transit) Jupiter or be hidden (occulted) and their shadows fall on the cloud tops of Jupiter, visible in modest (3-inch, telescopes as inky black dots slowly drifting across the face of the planet.
Saturn Ring Angles – The rings of Saturn are visible in the smallest telescope at 15x while a good 60mm scope at 50x-75X can show them as a separate structure detached on all sides from the ball of the planet. Saturn has a tilt of 27 degrees, so the rings are tilted at an angle of ~27 degrees to the more visible rings orbiting above Saturn’s equator making them appear different from year to year as observed from Earth. Earth passes through the ring plane every 13 to 15 years, which is around every half Saturn year, making them appear sliver thin….in fact this was going on when Galileo first observed them, next pass occurs in March of 2025.
Light Pollution and the Bortle Scale – a nine-level numeric scale that measures the night sky’s brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by light pollution. John E. Bortle created the scale and published it in the February 2001 edition of Sky & Telescope magazine to help amateur astronomers evaluate the darkness of an observing site, and secondarily, to compare the darkness of observing sites. The scale ranges from Class 1, the darkest skies available on Earth, through Class 9, inner-city skies. It gives several criteria for each level beyond naked-eye limiting magnitude (NELM)
Zodiacal Light & the Gegenschein – Zodiacal light is a faint, diffuse, and roughly triangular white glow that is visible in the night sky and appears to extend from the Sun’s direction and along the zodiac, straddling the ecliptic. Sunlight scattered by interplanetary dust causes this phenomenon. Zodiacal light is best seen during twilight after sunset in spring and before sunrise in autumn, when the zodiac is at a steep angle to the horizon. However, the glow is so faint that moonlight and/or light pollution outshine it. Gegenschein countershine’) is a faintly bright spot in the night sky centered at the antisolar point. The backscatter of sunlight by interplanetary dust causes this optical phenomenon, also called counterglow.