Figure 17-22.  At night, the horizon may be hard to discern due to dark terrain and misleading light patterns on the ground.  parallel to another’s course. To determine the direction of  flight, pilots should observe aircraft lights and their relative  position to the horizon. If the intensity of the lights increases,  the aircraft is approaching; if the lights dim, the aircraft is  moving away.  This illusion results from viewing a source of light that is  increasing or decreasing in luminance (brightness). Pilots  may interpret the light as approaching or retreating.  Size-Distance Illusion  Fascination (Fixation)  Flicker Vertigo  Night Landing Illusions  Enhanced Night Vision Systems  This illusion occurs when pilots ignore orientation cues and  fix their attention on a goal or an object. Student pilots tend to  have this happen when they are concentrating on the aircraft  instruments or attempting to land. They become fixated on  one task and forget to look at what is going on around them.  At night, this can be especially dangerous because aircraft  ground-closure rates are difficult to determine, and there may  be minimal time to correct the situation.  A light flickering at a rate between 4 and 20 cycles per  second can produce unpleasant and dangerous reactions. Such  conditions as nausea, vomiting, and vertigo may occur. On  rare occasions, convulsions and unconsciousness may also  occur. Proper scanning techniques at night can prevent pilots  from getting flicker vertigo.  Landing illusions occur in many forms. Above featureless  terrain at night, there is a natural tendency to fly a lower­ than-normal approach. Elements that cause any type of  visual obscurities, such as rain, haze, or a dark runway  environment, can also cause low approaches. Bright lights,  steep surrounding terrain, and a wide runway can produce the  illusion of being too low with a tendency to fly a higher-than­ normal approach. A set of regularly spaced lights along a road  or highway can appear to be runway lights. Pilots have even  mistaken the lights on moving trains as runway or approach  lights. Bright runway or approach lighting systems can create  the illusion that the aircraft is closer to the runway, especially  where few lights illuminate the surrounding terrain.  Prior to flying at night, it is best to learn and know the  challenges of the area in which you are flying in. Study the  area and know how to navigate your way through areas that  may pose a problem at night. For example, many areas near  water may be obscured by low lying clouds or fog. To help  deal with this type of situation, it is important to have a plan  before you leave the ground. In the daytime, fly the routes  and passes that you will be flying at night and determine the  minimum altitude you are willing to use at night. If weather  prevents you from maintaining the altitude that you planned,  make a decision early to turn 180° and land at an alternate  airport with better weather conditions. Always consider  safer alternatives rather than hope things will work out by  taking a chance.  Pilots who fly at night should strongly consider oxygen  supplementation at altitudes and times not required by the  FAA, especially at night when critical judgment and hand-eye  coordination is necessary (e.g., IFR) or if he/she is a smoker  or not perfectly healthy.  Synthetic Vision Systems (SVS) and Enhanced Flight Vision  Systems (EFVS) are two systems that can improve the safety  of flight at night. The technology of both is evolving rapidly  and being used more and more.  [Figure 17-23]  17-27  Apparent horizon Actual horizon 