Twilight is that magical time of day when a glow nonetheless pervades the air although the sun has set. It occurs twice a day, earlier than dawn and after sundown. Earth’s environment scatters the sun’s rays to create the colours of twilight. On worlds with no atmospheres, such because the moon, the sky falls instantly dark after the sun units.
And, in the event you may see twilight from outer space, you’d discover that it isn’t marked by a pointy boundary on Earth’s floor. As an alternative, the shadow line on Earth – typically known as the terminator line – is unfold over a reasonably extensive space on the floor and reveals the gradual transition to darkness all of us expertise as night time falls.
3 varieties of twilight
Astronomers acknowledge three completely different phases of twilight: civil, nautical and astronomical. All three phases occur twice in a 24-hour interval, between day and night time after which repeating in reverse order between night time and day.
Stage 1: Civil twilight
Let’s think about the phases of twilight as occurring after sundown. Take into account that they’d reverse their order at dawn. Civil twilight begins the second the sun slips beneath the horizon. The official definition of civil twilight is the time from when the sun disappears till the sun’s heart is 6 degrees beneath the horizon. A measurement of 6 levels of sky is a little more than three fingers held at arm’s size.
Throughout civil twilight, there’s sufficient gentle to see, however individuals activate their lights to drive a automobile, and the streetlights are beginning to come on. The brightest planets seem throughout civil twilight.
For mid latitudes, civil twilight lasts a bit longer in summer season and winter and is a bit shorter in spring and fall. In spring and fall, the sun rises and units extra instantly within the east and west. Due to this fact, it makes a straighter path downward (or upward), reaching the 6 diploma mark in a shorter time period. In summer season and winter, the sun arcs throughout the sky, chopping throughout the horizon at an angle. This angle is extra pronounced in summer season, which is why civil twilight lasts the longest in summer season. Civil twilight in mid latitudes can final, on common, 1/2 hour.
Examine this to tropical areas. On the equator, the size of civil twilight hardly varies. The sun across the equator makes a path throughout the sky that cuts cleanly down towards the horizon at sundown in a virtually perpendicular style. Due to this fact, the sun and its rays disappear quicker, giving equatorial areas a shorter twilight than greater latitudes. Close to the poles, twilight occasions final for much longer.
Stage 2: Nautical twilight
Within the night, nautical twilight takes over the place civil twilight ends. The definition of nautical twilight is the time interval when the middle of the sun is 6 levels beneath the horizon to 12 levels beneath the horizon. You possibly can bear in mind the identify “nautical” as a result of it ends when the distant line between sea and sky is now not distinguishable. Additionally, extra brilliant stars seem throughout this time, which was necessary within the early days of navigation. When nautical twilight started, sailors may use the celebrities as directional cues.
Throughout nautical twilight, terrestrial objects are seen, however you want synthetic lights to hold on out of doors actions.
For polar areas, the summer season sun doesn’t get greater than 12 levels beneath the horizon. Due to this fact, these areas have nautical twilight all night time lengthy, by no means reaching astronomical twilight or total darkness. For mid latitudes, nautical twilight can final from about 1/2 hour in spring, winter and fall, to about 45 minutes in summer season.
Stage 3: Astronomical twilight
The darkest twilight stage is astronomical twilight. The definition of astronomical twilight is the time period when the middle of the sun is 12 levels beneath the horizon to 18 levels beneath the horizon. You in all probability don’t even discover any illumination left within the sky at the moment.
For stargazers, that is the time when fainter stars, clusters and different sky objects seem and turn into good observing targets.
In mid latitudes, astronomical twilight can final about 1/2 hour from fall by means of spring however as much as an hour in summer season. Astronomical twilight begins about an hour to 1 1/2 hours after sundown for mid latitudes. So, as a rule of thumb, in the event you’d like to look at one thing within the night time sky that isn’t notably brilliant, you must wait about 90 minutes after sundown earlier than you begin observing.
Photograph gallery from the EarthSky neighborhood
Backside line: Twilight is that magical time between daylight and darkness. Astronomers, the consultants on nighttime, acknowledge three phases of twilight.
