how is that possible without radar?
As others have already said, ATC Towers don't provide airborne separation services to VFR aircraft. What they do provide is SEQUENCING to the runway and RUNWAY separation. RUNWAY separation is the reason why clearances are required to taxi, takeoff, or land at a towered airport.
Towers without a radar display rely on pilot position reports and visual observations. Take the time to visit your local tower for an hour, or so. They'll show you how it is done. AIM 4-1-6 discusses pilot visits to ATC facilities.
That is a frightening thought while flying in IMC.
The majority of the Earth's service is non-radar. IFR procedures were developed without any dependence on radar using airways, fixes, and position reporting. Radar allows for reduces separation standards, random routings, and vectors which greatly increases system capacity.
AIM 5-3-2 covers position reporting, 5-3-3 is additional reports, 5-3-4 is airways and route systems, and it continues through the end of Section 5-3. Those sections describe the en-route procedures upon which non-radar separation are based.
I can't, unless you can tell me whats the difference between radar service and traffic separation?
Start with AIM 4-1-18 Terminal Radar Services for VFR Aircraft.
When separation services are provided, ATC issues clearances which provide for the applicable minimum separation distance. This comes in the form of routing, altitude, speed, or heading assignments. When ATC is providing advisory service they are not issuing clearances nor attempting to maintain a specific minimum distance of separation.
From the P/CG:
RADAR FLIGHT FOLLOWING− The observation of the progress of radar identified aircraft, whose primary navigation is being provided by the pilot, wherein the controller retains and correlates the aircraft identity with the appropriate target or target symbol displayed on the radar scope.
RADAR SERVICE− A term which encompasses one or more of the following services based on the use of
radar which can be provided by a controller to a pilot of a radar identified aircraft.
a. Radar Monitoring− The radar flight-following of aircraft, whose primary navigation is being
performed by the pilot, to observe and note deviations from its authorized flight path, airway, or route.
When being applied specifically to radar monitoring of instrument approaches; i.e., with precision
approach radar (PAR) or radar monitoring of simultaneous ILS/MLS approaches, it includes
advice and instructions whenever an aircraft nears or exceeds the prescribed PAR safety limit or
simultaneous ILS/MLS no transgression zone.
(See ADDITIONAL SERVICES.)
(See TRAFFIC ADVISORIES.)
b. Radar Navigational Guidance− Vectoring aircraft to provide course guidance.
c. Radar Separation− Radar spacing of aircraft in accordance with established minima.
RADAR TRAFFIC ADVISORIES− Advisories issued to alert pilots to known or observed radar traffic which may affect the intended route of flight of their aircraft.
(See TRAFFIC ADVISORIES.)
SEPARATION− In air traffic control, the spacing of aircraft to achieve their safe and orderly movement in flight and while landing and taking off.
(See SEPARATION MINIMA.)
(See ICAO term SEPARATION.)
SEPARATION MINIMA− The minimum longitudinal, lateral, or vertical distances by which aircraft are spaced through the application of air traffic control procedures.
(See SEPARATION.)
