Observing stars with a telescope in the early evening is possible, but the experience depends heavily on the amount of light still present in the sky. Effective telescopic viewing hinges on understanding the transition from daylight to deep night and the specialized function of the telescope itself. This helps set realistic expectations for what can be seen immediately after the sun dips below the horizon.
The Transition from Day to Night
The period commonly referred to as “evening” is categorized into three distinct phases of twilight, defined by the sun’s position below the horizon. Civil twilight begins at sunset and lasts until the sun is six degrees below the horizon. During this phase, there is still enough natural light for most outdoor activities, and only the brightest planets and stars are visible to the unaided eye.
Nautical twilight follows, continuing until the sun reaches twelve degrees below the horizon. During this phase, the horizon is still dimly visible, a condition historically useful for sailors. Stars and planets brighter than magnitude three are generally visible at this point.
Optimal viewing conditions for a telescope begin only after astronomical twilight, when the sun is eighteen degrees below the horizon. Before this stage, the sky still scatters sunlight, which reduces the contrast necessary to see faint objects. This residual blue light effectively washes out the delicate glow of distant stars and nebulae.
The Primary Function of a Telescope
The ability to see faint celestial objects in the early evening is not primarily due to magnification, which is often mistakenly considered the main purpose. Magnification simply enlarges the image, but also spreads the collected light, making faint objects appear dimmer. The most important feature of any telescope is its light-gathering power, the capacity to collect photons from distant sources.
A telescope acts like a larger version of the human eye’s pupil, collecting far more light over a given area. This capacity is strictly a function of the diameter of the objective lens or mirror, known as the aperture. For example, a 20-centimeter aperture collects four times the light of a 10-centimeter aperture, because light-gathering power increases with the square of the diameter. Collecting more photons brightens the image, making it visible against the residual background light of the twilight sky.
Reflector telescopes use a large primary mirror to gather light and are typically built with much larger apertures than refractors, which use a lens. Since the mirror is less expensive to produce in large sizes, reflectors are often preferred for maximizing light gathering for deep-sky observation. Refractor telescopes provide sharper, higher-contrast images, especially for planets, but are limited in aperture size and thus gather less light than comparable reflectors.
What Telescopes Reveal in the Early Evening Sky
Even before the sky reaches the full darkness of astronomical night, a telescope can reveal numerous targets that overcome the atmospheric light. The most successful objects for early evening viewing are those with high surface brightness, meaning they concentrate light into a small area. The Moon is the prime target, as its surface is extremely bright and its features, like craters and mountains, are easily visible even during civil twilight.
Bright planets are also excellent targets because their proximity makes them highly luminous, allowing details to cut through the sky glow. Planets like Venus and Jupiter are often visible soon after sunset; Venus is sometimes so bright it casts shadows. Saturn’s rings and Jupiter’s atmospheric belts become discernable features well before the fainter background stars appear.
While individual stars are easily seen, deep-sky objects (DSOs) like galaxies and faint nebulae remain challenging until the sky is truly dark. However, some bright star clusters, such as the Pleiades, are luminous enough to be viewed in the early evening. Using a telescope during twilight is primarily about observing bright, non-stellar objects that can outshine the remaining daylight, rather than searching for the faintest, most distant stars.