Astronomy

Asronomy has been foundational to the development of perspective methods/systems. Both fields are closely intertwined, forming a single body of scientific work throughout history.

A wide range and huge number of different kinds of astronomical perspective have been employed throughout time, contributing to the development of civilisation (navigation, farming, astronomical observations, etc), and science. Examples include seasonal variations, sundials, navigation and cartography, solar-system and deep space studies, astronautics, etc.

Let us explore some of the links between perspective and astronomy.

Perspective in Astronomy

Perspective has several meanings as applied to astronomy, as listed below.

Perspective-Taking (Geocentric vs. Allocentric): Educational astronomy uses “perspective- taking” to help users switch between a geocentric (Earth-centred) and an allocentric (space-based/orbital) perspective to understand seasons, moon phases, and planetary motion, etc.
The Celestial Sphere: A conceptual, imaginary, and concentric sphere with an arbitrarily large radius, centred on the Earth or the observer, onto which all celestial bodies are projected to measure their positions.
Parallax: An observational tool based on perspective, where the shift in position of nearby stars/planets relative to distant background stars is measured from different points in Earth’s orbit (or, as with New Horizons, from interstellar distances).
Cosmic Perspective: A conceptual viewpoint, often adopted in cosmology and outreach, that emphasises understanding the scale of the universe and humanity’s place within it.

Celestial Sphere

“Celestial perspective” refers to the process and outcome of obtaining a view/image of the celestial sphere, which is an imaginary, vast, concentric sphere with an arbitrary or infinite radius centred on the Earth, upon which all celestial bodies (stars, planets, Sun, Moon) appear projected. It is a vital tool for positional astronomy and navigation, allowing astronomers to map the sky using a coordinate system similar to Earth’s latitude and longitude.

The celestial sphere is an imaginary sphere surrounding Earth used to model the sky’s apparent motion, aiding astronomers and navigators in visualising celestial object positions.

Principles

The celestial sphere is concentric with Earth and has an arbitrarily large radius.
The celestial sphere’s rotation causes the stars to rise in the east and set in the west.
The celestial sphere’s poles correspond to the Earth’s poles, and the celestial equator
corresponds to the Earth’s equator.
The celestial sphere is the surface on which all objects in the sky are projected.

Applications

Used to establish coordinate systems for marking the positions of celestial objects.
Used to model the apparent motion of the sky, which helps explain the seasons.

The celestial perspective concerns capturing views/images of objects or matter existing, and processes taking place, at astronomical distances in outer space; which has implications for the perception of the apparent and also implied object forms, distances, sizes, locations, angles, etc.

In terms of perspective processes/outcomes involved, to begin, we must consider the class of perspective, whereby Astronomy is broadly divided into observational astronomy (using telescopes to gather data across electromagnetic spectrums like radio, optical, X-ray, and infrared) and theoretical astronomy (using models to understand celestial phenomena). We also have gravitational telescopes and particle detectors for astronomical observations.

In terms of optical and visual perspective (2nd type), we note that celestial or astronomical perspectives have different kinds; from observations of seasons (seasonal perspective), to planetary motions, stellar distances (for example parallax (A)), shape/morphology of galaxies (galaxy perspective), etc.

A key difference from Earth-bound perspective, is that astronomical objects – and especially stars are incredibly large and distant – and that A) the object (e.g. a star) may not be resolved sufficiently to measure size/distance directly, and B) the light rays emanating from such an object (e.g. a star) are effectively parallel and unresolved, one relative to anther, and C) stellar objects or stars are typically not related in structural terms – or at the same distance from Earth, and thus within each constellation the stars are at vastly different distances and so only apparently visually connected.

Accordingly, typical convergence, as seen with linear perspective (size/distance law), may not be applicable in many cases. Unless the imaged stellar objects are at the same approximate (or relative) distance, or else related together in some other structral way, the rules of degradation of form perspective will not apply; but sometimes they will apply, as with a true circular galaxy that appears elliptical when seen edge on (aspect foreshortening).

Armillary Sphere

An armillary sphere (variations are known as spherical astrolable, armilla, or armil) is a model of celestial objects, featuring a framework of rings that represent celestial longitude, latitude, and other astronomical features

Astrolabe

The Astrolabe is an ancient astronomical instrument, originally developed in China and Greece, serving as a handheld model of the universe and an inclinometer for astronomical calculations.

Several types of astrolabe are known, including:

Circular astrolabe
Prismatic astrolabe
Spherical astrolabe
Stereographic astrolabe
Universal astrolabe or saphea
Linear astrolabe
Mariner’s astrolabe
Planispheric astrolabe

Astronomical Ring

Astronomical rings (Latin: annuli astronomici), or Gemma’s rings, are an early astronomical instrument. It consists of three rings, representing the celestial equator, declination, and the meridian. It can be used as a sundial to tell time, if the latitude and season are known, or to tell latitude if the time is known or observed (at solar noon). It is a simplified, portable armillary sphere, or a complex type of astrolabe.

Celestial Globe

A celestial globe is a 3-D, inverted, geocentric model of the night sky, featuring stars, constellations, and the ecliptic mapped onto a sphere with Earth at the centre. Unlike terrestrial globes, celestial globes often depict the heavens as viewed from outside and are often used for teaching astronomy, navigation, and identifying celestial objects. Modern celestial globes often include illuminated features, digital or satellite data.

Celestial globes depict the positions of stars, excluding the Sun, Moon, and planets. They often present a “handedness” issue; if the stars are positioned accurately, the constellations appear reversed due to the difference between the Earth- centered gnomonic projection and the outside-view orthographic projection. To counter this, some globes are made in mirror image form, while modern versions may be transparent, introducing distortions. Opaque globes can also place constellations correctly but may appear as mirror images from outside, often requiring the use of a mirror for accurate viewing.

Astronomical Calculator

Refers to one of several kinds of astronomy calculator used for simplifying and automating complex astronomy related calculations. One example are star charts that calculate a celestial objects position in the night sky.

Factor’s affecting star position include:

Earth’s rotation: stars appear to cross field of view of telescope
Earth’s orbit around the sun: background position of stars changes each night (slightly)
Axial precession and nutation: Earth’s axis of rotation changes slowly over long period of time
Aberration and parallax: Effects of the Earth’s orbit around the sun
Proper motion: motion of the individual stars

Spherical Astronomy

Spherical astronomy (or positional astronomy) is the branch of astronomy that uses spherical trigonometry and observational methods to determine the positions and apparent motions of celestial objects on the imaginary celestial sphere. It is essential for navigation, timekeeping, and locating objects, relying on coordinates like right ascension/declination and altitude/azimuth.

Ecliptic

The ecliptic is the imaginary plane that describes Earth’s orbit around the Sun. It’s also the apparent path of the Sun across the sky throughout the year.

Applications

Astronomy: The ecliptic is a key line used to divide the night sky. It’s also used to make measurements in astronomy.
Astrology: The constellations of the zodiac are arranged along the ecliptic.
Calendar-making: The ecliptic was a central concept in ancient sciences, including calendar-making.

Properties

The ecliptic is elliptical, not perfectly circular.
The ecliptic plane is tilted at about 23.5 degrees relative to the celestial equator.
The ecliptic and the celestial equator meet at the equinoxes.
The ecliptic is named for the fact that eclipses can only occur along it.

Solar system

The planets stay close to the ecliptic, but they don’t always move in the same direction (as viewed from an Earthbound viewpoint or perspective).
The planetary bodies in our solar system were formed from the Sun’s spinning, flattened, protoplanetary disk.

Astronomical Projection

Astronomical projection is a key component of spherical astronomy and cartography used to represent the 3-D celestial sphere (containing stars, planets, and galaxies) onto a 2-D plane, such as a star map, photograph, or computer screen. These projections often use specific perspective models to balance, shape, and represent distance and area for observers on Earth or in space.

Types of Astronomical Projections

Stereographic Projection: The most common projection used for all-sky maps and planispheres. It is conformal, meaning it preserves angles and shapes of constellations, though it distorts area near the edges.
Gnomonic (Rectilinear) Projection: Used for narrow-field photography (like smartphone cameras) and meteors, as it projects straight lines on the sky as straight lines on the map. It is often used for maps centred on specific objects.
Orthographic Projection: Represents the celestial sphere as seen from an infinite distance, depicting a hemisphere as it appears from space, with high distortion near the edges.
General/Tilted Perspective Projection: Simulates a camera view from a finite distance above the surface (e.g., Earth-viewing satellites or NASA World Wind).

Parallax

Parallax is the apparent displacement or shift in position of an object when viewed from two different, non-aligned lines of sight. It is a foundational method for calculating distances, particularly in astronomy and computer vision.

Key aspects of parallax include:

Distance Measurement (Stellar/Astronomy): Nearby objects have a larger parallax (shift) than distant ones. Astronomers use this to determine the distance to stars by observing them from opposite sides of Earth’s orbit (annual parallax).
Depth Perception (Stereopsis): Human eyes use parallax to perceive depth, as each eye views objects from slightly different angles.
Motion Parallax: Objects closer to an observer appear to move faster than distant objects when the observer is in motion.
Parallax Error in Measurement: In instruments, this error occurs when a pointer and scale are not in the same plane, causing a different reading based on the viewing angle (e.g., in a car speedometer or analogue gauge).
Visual Effects: Used in computer graphics and web design to create an illusion of depth by scrolling background layers at different speeds.
Imaging & Technology: Used in camera viewfinders, microscopes, and to correct image stitching in photography.

Astronomical Calculation
The distance to an astronomical object such as a star or planet can be calculated as the reciprocal of its parallax angle (d = 1/p), where d is the distance in parsecs and p is the angle in arc-seconds

Perspective Axis Constellation

A perspective axis constellation graphic generally refers to a 2-D representation of 3-D star positions, or more technically, in the context of star mapping, a constellation graphic uses an equirectangular plot to map the 3-D celestial sphere onto a 2-D surface.

Declination vs. Right Ascension: These plots use Right Ascension for the x-axis and Declination for the y-axis to represent the positions of stars as seen from Earth.
Mapping 3-D to 2-D Illusion (constellation patterns): Astronomical Projection graphics show how we perceive stars in a 2-D pattern, even though they are at widely different distances in space.

Gravitational Lensing

Type of astronomical perspective whereby a distant celestial object is viewed along a special line-of-site that has another massive celestial object directly in front of the first object; whereby the second object acts as gravitational lens that bends light from the first object in a similar fashion to bi-convex lens, magnifying the image, and thus making the first (very distant) object seem closer than in reality (or making said object visible when it would not have been without the effects of the gravitational lens).

Galaxy Perspective

In astronomy, when a true circular galaxy is viewed from an angle (not face-on), the top and bottom are “squished” together, making it appear as an ellipse due to aspect foreshortening.

Planisphere

A planisphere projection is a flat representation of a sphere, such as the celestial sphere, on a plane. It’s a type of map projection that’s used to create star charts.

Explanation

A planisphere is a map of the sky, or celestial sphere, projected onto a flat surface.
It’s often used in astronomy to show the stars and other celestial objects visible from a specific location at a given time.
The projection process can distort the scale, area, and distance of the celestial sphere.
Planispheres often have adjustable circles or other appendages to help show celestial phenomena.

Using a planisphere

To use a planisphere, hold it as if you were outside observing the sky.
You can set the planisphere to the time, day, and month of your birth to find the stars or constellations visible at that time.

Planetarium

A planetarium is a theatre for educational and entertaining astronomy shows, featuring a dome where realistic projections of celestial objects simulate their motion. Projections in planetariums can be made using various methods, including star balls, slide projectors, and full spherical-dome systems. These systems simulate the night sky from any latitude and can reflect any time period.

Planetariums range from large domes, like the 37-meter one in St. Petersburg, to portable three- meter versions. Their origins trace back to ancient Greece.